Thanks for your suggestions. It does seem that if (via the OPT button on the notation screens) the triplet resolution is checked then the quarter note triplets do display and print correctly. If, however, (via the OPT button) triplet resolution is unchecked but the resolution for the beats in the particular measure are set to 3 then, as I mentioned in my original post they often don't necessarily display and print correctly.

Checking the triplet resolution via the OPT button seems to solve the original problem but it creates another problem which is the converse of the original. I find that where other parts of the song contain eighth notes (entered exactly at the correct times) some of these display and print incorrectly as tied eighth note triplets (but others in the same song display correctly).