Muffled tapes

[RobF]

A quick question if I may - I know a lot of our tape rippers are
knowledgeable in this field..

If we have an old tape and play back is muffled (sounds almost underwater)
what are the general causes of this and is there any way to fix it?

I noticed on a lot of my last batch of L tapes that the start of the tape
(first minute or so) was often muffled but it soon righted itself.
Sometimes it would be muffled but if I pressed stop then start again it
became unmuffled immediately.

Any thoughts? I know we've discussed azimuth at length before but does this
sound like the same thing?