Good points from JDFanatic. I'd like to add a bit more to what he says. Make absolutely sure the alignment V-groove on the "C" Grips have dropped completely and evenly on to the locating tabs on the FEL mounts.
Also if the MMM is on the tractor, the front mounting hoop release pin will sometimes interfere slightly during re-mounting of the FEL.
I have also had, on more than one occasion, a very difficult time re-attaching the hydraulic hoses. Typically, I'll remove the FEL when it's cool, then it sits in the sun and builds up quite a bit of pressure. You have to release the pressure on the hoses by pushing in the "valve" pin at the end of the hose. This can be very difficult to do if there is alot of pressure built up.
Once you know all of ins and out, you'll find it is really a breeze to mount and dismount the FEL.