The hydraulic deck lift is a really nice feature on the kubota zd21. But lately its getting to the point where its really slow coming up after its been running for a few minutes. And today it wouldn't come up at all after about 30minutes of mowing. When I first start it for the first time of the day it raises and lowers without any problems. But the longer it runs (15mins) the slower it gets. After about 30minutes or so it won't lift at all unless it is assisted. I checked the fluid level and it was fine. What else should I do?????