As others have guesstimated, the powder/bullet combination isn't generating enough velocity/recoil to reliably cycle the slide every round.
What other powders do you have available?
As to LOA...every bullet/gun combination is a law upon itself. For an auto loader one has the following to consider:
1) Maximum magazine LOA
2) chamber/lead max LOA
3) Best LOA for reliable feeding
Test by:
1) Magazine...can the magazine be filled without the rounds binding
2) Take the barrel out and drop a round into the chamber. Does it headspace correctly or is the case rim sticking up above the barrel hood.
3) If #1 and #2 check out ok, fill the magazine preferably with dummy rounds and SLOWLY hand cycle them from the magazine to the chamber in one smooth motion. Then reload the magazine and pull the slide back smartly letting it go at the end of the rack. If both tests run ok, especially the slow cycling, then the rounds should run IF there is enough velocity to properly cycle the slide with the bullet weight chosen.
As to the Lee FCD...I am a believer. The die not only Taper Crimps the case mouth but a carbide ring inside the die makes sure that every round is sized back factory spec.
From Lee Precision:
A carbide sizer inside the Carbide Factory Crimp die post-sizes the cartridge while it is crimped so every round will positvely chamber freely with factory like dependability. The adjusting screw quickly and easily sets the desired amount of crimp. It is impossible to buckle the case as with a conventional bullet seating die. Trim length is not critical so this extra operation takes less time than it would if cases were trimmed and chamfered.
Revolver dies roll crimp with no limit as to the amount. A perfect taper crimp is applied to auto-loader rounds. The crimper cannot be misadjusted to make a case mouth too small to properly head-space. A firm crimp is essential for dependable and accurate ammunition. It eliminates the problems of poor ignition of slow burning magnum powders.
Hope this helps...
Bob
this, plus one other thing maybe. I have got into the habit of miking the taper crimped round to get an external diameter at the case mouth the same as a factory round to let me know if i have crimped enough or too much.
