I couldn't comment on the guitar, but the C chord seems to cause a lot of problems for beginners.

The main causes for problems that I have seen are:

twisting the wrist or hand into a poor position

flattening one or more finger so that it mutes adjacent strings

pulling fretting-hand elbow into the body

pressing too hard, so that it forces strings to move

tension in hands, wrists, and arms which makes playing generally more difficult.

The other thing is, like all chords and chord changes, it takes time to get it locked into muscle memory.