Low power hydro issue

I am trying to get some direction on a hydraulic issue. While digging a few Perc trenches, I noticed that the power to the boom cylinder faded on the third trench (after about 40 minutes). I had the boom and dipper straight, with a full load of dirt in the bucket, and I could not lift the hoe with the boom cylinder. Only when I pulled in on the dipper could I lift the bucket out of the hole. Can someone give me some things to try to diagnose if it is a boom cylinder that needs repaired or something else. Thanks Reylocs