Please help I have JD 60" med mount mower deck I can't keep the drive belt on I place new belt on and same problem any way to adjust the belt?

Did you replace with OEM? Or generic? Sound like it's not the correct size or not routed properly

You need to be sure you understand where the spring loaded tensioner pulley fits in this sequence. You have to pull back on the tensioner as hard as you can and then go around that pulley so when you release tension it will snap tight. There is some adjustment on the spring but it doesn't normally need much, new OEM belts are tight.

ballengerb1 and esquire1 is correct, it seems as though you are using an aftermarket belt. Sometimes you get a better deal to only find out it doesn't work. And ballengerb1 is also correct, new oem belts will be tight, and be sure you are routing the belt correctly.