Tamiya Masking tape leaving marks on Tamiya flat paint??

You should find that the streaks disappear under a coat of gloss.

I always apply a coat of Future  in between colours, as not only does this prevent the tape leaving a mark on the underlying paint, but also the tape adheres better to a gloss surface, giving you sharper masking.

Karl 

How long had the base coat been dry before you applied the tape? The only thing I can think of is that the base coat may not hae been fully cured before you masked up. I would suggest leaving it to cure at least overnight before masking up.