I have a EOS Rebel T3 along with the EFS 18-55mm lens and the EF 75-300 lens. Recently the EF 75-300 lens has stopped attaching properly to the camera while the EFS 18-55mm lens still attaches fine. I don't see any damage to the lens but thought I would check and see if anyone else has run across this problem?
One thing I would suggest looking into specifically are the screws on the silver lens mount. If one of those screws is loose it may be protruding enough to cause an issue. you can lightly tighten these screws with a small phillips head screwdriver. Otherwise, the lens might need service to address something else with the mount.
This is a cheap and cheerful kit lens of, at best, meadocre quality. It is not worth servicing: either ecomomically or quality-wise.
I would suggest replacing it with thee excellent. EF-S 55-250 IS STM or, if you ant the reach and are ptepared to invest a bit more, the excellent. EF 70-300 IS USM lenses. Either the Mk1 OR 2 versions are great lenses. See my review of them Here
Suggest you keep an eye on the Canon refurb lens site at This Link