Financial Difficulty Due To Employer Withholding Mistake

My first two checks after transferring were about $100 short because my employer took out that amount for AZ state tax.It should have been less than $20. I can't seem to catch up. I need to register my car, etc.. When asked if they were going to reimburse me (fix it), they said it had already been sent in. I know they are not sent in more than quarterly. What should I do?