I had the same issue with my 45s a couple of years ago. slid the forks under a pile of slabs, and went to curl back, and just went into bypass. I though that was strange until I decided to just raise the pile, and instead of the loader coming up, the back tires did.
If you have a heavy load out toward the end of the forks, that is a lot of leverage for the small surface area on the retract side of the cylinders to overcome.