A gall-midge affecting Common Cat's-ear (Hypochaeris radicata) casuing the flowers to be distorted and some florets to be stunted.  Small yellow larvae feed inside the receptacle.   The larvae can jump, and several may be found inside each flower head.  Note that much larger white maggots or pupae of a different species of fly may also be found. 

You must check for the larvae and provide close-up photographs of them. Also check they are jumping - remove tham and watch for a while!