The most obvious problem would be an incorrect shutter speed when flash is used. If the shutter speed was too high the flash and shutter can't synchronize - the result is a black strip or entirely black frame on the negative/slide, depending on how far off the sync was.

Check the manual to determine what the maximum flash sync is for your camera - it's probably around 1/125 second. Make sure the shutter speed is at or below that speed when using flash.

If you weren't using flash when these problems occurred then there might be something wrong with the shutter but I'd first want to consider other errors such as setting the exposure incorrectly for a given situation. This may explain totally blank frames, but not partially blank frames - the latter would most likely be due, again, to incorrect flash sync or a shutter problem.