When you drag your Audio Clip to any empty Audio Track, hold down the Ctrl modifier key, to Insert Edit, rather than Ripple Edit.
You can only Delete and Close Gap, if there are no Clips above, or below that Clip/Gap to be Deleted.
Also, MP3's can be problematic. This ARTICLE goes into more detail. However, from your description, I think that it is difference between Insert Edit and Ripple Edit.
Brilliant. Many thanks, that solved my problem. I'll read with interest article about MP3s. Tim
Great news! Thanks for reporting.
It's interesting, at least to me, that PrE chose the Ripple Edit as the default, where Insert Edit is the default in PrPro. The Ctrl modifier key toggles to the other method in each program, but they are just the opposite of each other. I often have to stop and think which program I am in, and watch the Cursor, to see which mode I am in. Guess that the PrE developers figured that the average user would want Ripple, while the PrPro developers decided their user-base would want Insert.
For the MP3's, many work fine. I do not like the MP3 compression, so avoid it, should I be creating the Audio. I stick with the larger, but higher-fidelity PCM/WAV @ 48KHz 16-bit. When one has an issue with an MP3, converting, with, say Audacity, to PCM/WAV w/ those specs., usually solves the issues. I rather do that out of habit, problems or not, and only use MP3's for some SFX files, which are small. I just never use MP3's for music, or for dialog.
Good luck and happy editing,