Can you tell me how to fix form 1040 line 15a and 15b? 15a contains the correct amount while 15b has a much smaller amount.

I received form 1099-R with box 1 and 2a containing equal amounts. Box 2b was checked. I converted part of a Rollover IRA to a Roth IRA. I expected to be charged a significant amount of tax but the taxable amount on line 15b is way too small.