I just took a look at my tapes and see the same thing. Some are a lot more shiny than others.
I tried one of the non-shiny ones in my tape machine and it worked fine - no problem.
I may well consider transfering some tapes to computer at some point. Most likely I'll never get round to it though
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I saw the date of your post was 6th December. 6th!?!? I thought it was the 5th! I'd better get to sleep.
Best of luck.