Curiously,
I never had this problem with the C trumpet, but this is probably because I use it less often on the orchestra than the B flat...I have an older colleague who is saying that this is coming of too much care for the horn
I took it as a joke than, but maybe he is right. If I was somewhere near Burgdorf I would go to Rene to fix the problem but Bulgaria where I am living and working at the moment is quite far away. Probably I should get an additional holder/stop as some schilke horn have (I think in particular about the schilke tunning bell horn in f which has that