Some coin mechs have an adjusting screw on the back that allows you to tweak it... others you have to investigate how it works and bend a few pieces to make sure it allows your quarters. Another option is to just purchase the proper coin mech.
Do you have a US or Canadian coin mech and are using the other coin? Do you have a token coin mech that's almost the same size? Or is it just worn out? Some adjustment is definitely possible but coin mechs aren't very expensive either if you continue to run into problems.