This is possibly an issue with dead keys, a way of using a "regular" keyboard to provide a way to generate diacrtics such as:
One way to test this would be to enter
'a in your terminal proper (rather than
vim within your terminal) and see what character is produced.
If it does produce a diacritic, that means the terminal may well be getting to the keyboard stream before
vim does, and all
vim will see is the diacritic rather than the expected key sequence.
If that is the case, you could opt to switch to another terminal which better allows you to control this, or there is at least one solution here which sneakily map the diacritic back into the correct key sequence that
vim will process.