Triplets

I had a score saved but somehow the file was corrupted. I couldn't open the file. I tried to reclaim the file by rescuing from a PDF file but many of the bars did not read correctly. The score is in common time (4/4) with triplets. For example a bar can have two triplets being written as six crotchets but of course their value is not six crotchets. The computer only allows me to do quavers. I know how to write in triplets but somehow the setting in this score has been changed and this basic operation is now impossible despite my many efforts to rectify it. Originally there were plus signs on certain bars and I was beginning to think that I had solved the problem of the time signature when I was able to delete these plus signs, but I am still having problems when triplets are required. I would really appreciate if you offer some guidance in giving me some options.