You may have too many filters. The mechanical pumps only put out about 3-4 psi and each filter causes a pressure drop, however I would think this would show up at higher RPMs. Also if you have a 2 micron as your primary that will have a larger pressure drop than a 20 micron.
In my opinion you just need two filters, the primary of 20 microns and then the secondary, which with my Yanmar is mounted on the engine, and it is a 2 micron. Anymore is over kill. Some people mount two primaries in parrallel so they can swing between them if one gets fouled while running.
My older 2GM starts first crank every time and it does not have glow plugs but I do give it half throttle.