If you lock up the mirror, the metering and AF systems receive no further light, so they can't function. You'll get a blinking "-no- AE" error message on the top LCD of the AFD, if you try to release the shutter while using one of the P/Av/Tv modes.
So if the camera is setting the exposure and/or focus for you, you must lock them in (press the AE lock button) before turning the M-Up lever. I bet that will solve your problem?
If you're using one of the M/X modes, setting the exposure manually, there should be no problem - it should fire with the M-Up set. Mine does.
Among the things I like about the original AFD is that once set, the AE lock and M-Up last as long as you please, and the M-Up lever, being mechanical, does not drain battery power in long exposures. This is sadly not the case with the later bodies (which also dumped the T setting, a no-battery-drain long exposure mode for film backs).
Ray