[drunken monkey]

Two things to beware when dealing with regular masking tape.

Grease does strange things to the gum.
Solvents from paints/aerosols does strange things to the gum.

You'll notice that Tamiya tape and higher grade painters tape (3M BLue tape for example) appears to be of a grease-proof like material. This is all probablity a seal on the tape to protect the gum from reacting with any other chemical.

Also, as mentioned before, sanding the surface prior to laying down primer helps. Not all plastic is the same. Just because it is labelled as for plastic bumpers, it doesn't automatically means it behaves the same on the plastic used for model kits.