I visually inspected the heads/tape path with a hand mirror when I received it.If the problem was buildup in the head gap, you might never see it.
If I have to resort to 'baking' a tape to get it to work, it's time to discard that tape.I would be careful about that. Once a tape is baked, its useful for some months before baking is needed again. If baking it properly doesn't sort it out, **then** its time to discard it.
To avoid having to bake tapes I have two tips. First, place the tape in a plastic bag when it goes in its box. Place a small bag of Silica Gel in the bag with it. Second, never store tapes in the basement- if in long storage, place them in the attic, or other location that tends to get warm (and tends to be dry). I store tapes I'm working with on a radiator in my house. The heat helps keep moisture from attaching to the polymers of the tape.