I usually put more into my IRA before April, in fact I believe I have done this EVERY year, once (2015) I did not and now the IRS sends me a bill???? I dont understand
Well if you deducted the IRA contribution and then didn't make the contribution then your return was incorrect. . As soon as you realized that you didn't make the entire contribution you listed on your return you should have amended the return asap. But you didn't .... So the IRS recalculated the return using the actual contribution that you made and sent you a bill for the balance due
As Critter#2 has suggested, examine the IRS notice carefully.