I would guess the major reason for nonsense mediated decay is that the protein is not properly folded and therefore the host scavenging system will degrade the unfolded or partially folded proteins. Each protein is going to have different kinetics for degradation depending its state so for some proteins you probably can easily see truncated species, whereas other proteins maybe not.

If you want to maximize the potential for seeing the truncated proteins I would be sure to use rapidly dividing cells so that you can capture recently translated protein (and not use old cells where the protein is likely degraded).