I also have a folding sluice which is convenient to travel on airlines to run in many different environments. Based on similar experience, my view is that the folding sluice set-up that comes standard requires a high water velocity to keep the first few riffles cleared out. The short length of the riffle section 28" makes this even more critical as there are only 8 riffles in my sluice. Several times using the sluice in slower moving streams, I have had to increase the drop from a generally recommended rate of 4" to close to 8" and still had buildup in the first riffles clogging them as the velocity in the early portions of the sluice were not enough to keep clear. This becomes difficult in relatively flat streams as well. I have recently purchased gold hog matting of downdraft and razorback designed matts to install in all three separate sections of the sluice in an alternating pattern. I am keeping them in three glued sections and then attaching them together on site for use when stream flow is low. I have mocked up the situation at home using a recirculating system, and at low to moderate flow rates, the matts seem to stay together without issues. I do not use the riffles or expanded metal in this set-up, as the matts are deigned to be used alone. I mixed some pay with known amount of varying sizes of gold and mixed with some local obtained typical dirt and the system was successful running three 5 gallon buckets of screened 1/8" material. (residual was run through gold cube with no additional picked up gold). Certainly you need to strongly considering classifying material to 1/8" or 1/4" for the folding sluice set-up with its short length. I am uncertain how this system will work in high velocity streams as gold hog provides two different matt combinations for their full length sluice for low and regular applications. Therefore, during my next few trips, I will be taking both the standard set-up and gold hog matts to compare when I get into higher velocity streams. Depending how things go, I may set-up a second set of gold hog matts deigned with higher flow matting they offer. The other obvious advantage using the gh matting is that I have increased the amount of capture by 8" in length versus the standard set-up, which increases capture length by close to 25%. Cost of matting shipped was ~$80.

The fact that the first and third riffles clog still sounds like a water velocity issue. Three factors (not the only ones) of water velocity in a sluice are speed of water entering the sluice, gravity force on water established by the drop of the sluice increasing the speed down through the sluice, and the reduction of water velocity by the riffles. If the velocity over a riffle is not sufficient it will clog. Once the first riffle is clogged, the water speed will not be slowed down as much as a clear riffle, so the water may now have enough velocity to keep the second riffle clear enough. The impact of the second riffle being clear slows the water back down enough to cause the third to clog. As the benefit of gravity moves water faster down the sluice length, it would appear that if the reminder of the riffles are remaining active enough to remain clear, you need to get the velocity faster entering the first riffle. Without seeing the set-up, it is difficult to be certain. You should be able to observe within the first scoop or two the action in the first riffle. If there is not an actively dancing particles and many of the obvious light particles and larger particles not being flushed down quickly, then the velocity is not high enough and you will need to increase the drop. I have discussed with several industry experts whether it would be an issue for the water to roll over the edges of the sluice in the lower section if that was required to get enough speed through the sluice, and was told that that was the better situation than allowing the sluice to clog. Therefore, if the lower section needs to be fully below the water surface to keep the first riffle from clogging, then that should be the approach. I don't have one, but there are simple devices sold by many sluice manufacturers that have a glass enclosed so you can place into the water and view the action in the riffle sections more clearly. 

     Well I got out one of my catalogs and looked at the Jobe folding sluice box.  In my opinion the sluice has an inadequate " Raceway " which can and will cause clogging especially if the material is to large or fed into the box to fast.

     The length of the raceway is what builds up head pressure from the top of the box thus creating the necessary force to move the majority of unwanted material over the riffle system and out the box.

     For example my 6 foot sluice box has 30 inches of raceway before the start of the riffle system and that is the sweet spot for this length box.  In the last 42 years I have never owned a store bought sluice box, for all of my boxes are homemade and field tested, so I know what happens if the riffle system is to long for the box.  For you see the water resistance created by the riffle system needs to be balanced to the length of the box for optimum stream performance.

     If I owned  the Jobe I would fab an extension to fit onto the top of the box to create the needed raceway length.  Also keep in mind that this info is for waterway sluice boxes only and does not apply to high bankers, dredges, or recirculating boxes.     Ron