Hooray for tax talk! The most likey thing going on is either:
1 - your withholding sheet is messed up and you need to update it
Or
2 - if you make OT or bonuses, it's possible your company isn't using the special bonus payment withholding rate, which can result in underpayment
Lastly, part of the last set of tax cuts that were passed included updating withholding tables to try to more closely match what people would actually owe. The idea was more money would go home in every paycheck with the trade-off being smaller refunds. Prior to that, the tables intentionally over withheld significantly. Most people making under $50k a year pay essentially nothing in federal income tax and most people under $100k pay very little (10-15% at most). It's SS and Medicare that get you.
Serious offer of help here too, you can PM if you've got questions. I love this crap, but don't want to clutter the thread too much.