What specific high build primer are you using? It sounds like you may be using a lacquer-based primer/surfacer. Are you thinning it as per the manufacturer's instructions? Most lacquer-based surfacers that I have used in the past are thinned 1:1 (100%), usually with lacquer thinner. What tip size are you using? The older non-hvlp suction feed guns mostly use a 1.8mm (0.070") tip for general purpose use. That would be the minimum that you'd want to use for a surfacer product. A 2.0 or 2.2 might be more appropriate for the product you are using. Do you have a tech sheet for it? For non-hvlp, try 40-50 psig pressure measured AT THE GUN with the trigger pulled. The fact that you are using a suction feed gun is not your problem. They worked fine in the past before gravity feed became the "norm". Just make sure you are following written manufacturer's instructions, both for the gun and for the exact paint product you are using.
Also make sure the vent hole in your suction feed cup cover is not partially plugged. Using suction feed guns out-of-position with a very fast dry primer or surfacer, can sometimes tend to plug that hole. It also goes without saying that the gun should be disassembled and carefully cleaned after each use. So make sure it is sqeaky clean. Check for any strainer that might be partially clogged on the bottom of the gun pickup tube. If you strain your paint before putting it in the cup, as you should be doing anyway, you don't need that strainer at all.