I've got a harmon muted trumpet that does 2 strange things:
it suddenly changes to another mute (I guess straight) at f1 and E-flat 1.
it suddenly drops in volume when f1 is repeated in one particular passage.
I had a similar problem with another instrument, a finger snap. Just wouldn't perform. At first it was very low in volume, then it dropped out completely. Solution: I deleted the entire staff, created a new instrument (this time handclap, but would probably work with finger snap or anything else), rewrote the part (without pasting the old one), and it worked fine. So I will try that method with the muted trumpet.