Teats collapsing on Dr Brown bottles

Hi everyone

I have been using dr brown bottles which has helped my lo tremendously with his gas how ever over the last couple of days the teats have been collapsing, has anyone else experienced this and know why it is happening? He is 9 weeks old and using size 2 as been having comfort milk. 

I have been a long time lurker on these forums so thank you to everyone that has helped me out when answering other people's questions that I've also wondered.

Thanks

Kirsty