2 things to check.
1. The linkage, make sure you don't have a bunch of saw dust under the linkage, you will be surprised at how a build up of dust will affect the movement.
2. Collector valve under the main bearing, or where ever it is on your truck, switches pressure between upper and lower controls. Is round and has a large snap ring on the bottom. Mine has 4 lines to it. Check that for any leaks, any what so ever. Cost about 65 bucks for a seal kit and will take a few hours to rebuild.
Neither one of those? Start pressure checking, you can get a proper gauge at any pro Hydraulic parts shop. I bought a good one, cost about 100
Its a pain checking them all, but it can help isolate the problem. Could be a 10 cent O-ring that is allowing blow back as well