Are they stock height springs? are you using and internal compressor with a stack of washers or a piece of pipe added to the top of the center threaded rod? The spring compressors are usually too long for the bottom area where the saddle is and the bolt hits before you can get it to let go (hence adding a spacers to the top). Of course you've got to line up the top of the spring inside the shock tower first.
Is your suspension at full droop? frame supported with jack stands and spindles/tires just hanging there?
I installed 1" drop 560 springs and they were 4" shorter than the stock springs I took out (stock 16", 560's 12" off the car.) I can remove/install my new springs w/o use of a compressor even, they slip out at full droop, this won't happen normally as the rest of the suspension connected - sway bar, strut rods, and shocks will prevent it.
'67 Coupe 289 stroked to 333, AOD, 9" w/3.50:1, PDB, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon