Hi Thank you for your question.
Collets should not wear out, or need replacing. Once locked into position they should become part of the mechanism only holding the attachment and spinning as part of the machine. Sometimes dust and dirt can get between the machine and the collet and this can make then not spin, or the collet locking nut can also get dirt in and not lock down correctly. Please trying cleaning the collet and locking mechanism. If this does not solve the problem please send me an email to firstname.lastname@example.org and i will contact you directly.