You guys are definitely doing something wrong.
Just flew an RNAV (RNP APCH) approach into MUCL RWY12 (Cuba) and while I thought it started descending too late, it did actually follow a profile to get the aircraft within minimums as published (253 ft above the field), at night.
Remember, these are NON-PRECISION approaches. You might be expecting an LPV (localizer performance with vertical guidance) which is kinda like a GPS-enabled ILS, but the A300 lacks the equipment and is not rated for such an approach, unlike some aircraft with the G1000 and G3000 systems that can fly them. Make sure you have also selected the CORRECT RNAV approach, as some are designed for slower, piston GA planes and have different profiles etc. I believe you should be entering only RNAV-1 Zulu approaches if more than one RNAV approach exists for the runway.
As per the manual your aircraft has to be fully configured for landing and stabilized BEFORE the FAF.