Many K-1's actually did not have to be sent to people until March 15th so you are probably correct in that it was not reported on the return filed.
You will need to Amend the return and after the Amended process is started go to:
Wages and Income
and scroll down to K-1 section to report.
For details on Amending a filed return see below:
Follow these directions:
3 weeks after mailing your amended federal return, you can start tracking it at the IRS Where's My Amended Return? site. Allow at least 16 weeks for the IRS to process your amended return.
Processing times for amended state returns vary from state to state. If it's been more than 12 weeks since you mailed your amended state return, and you haven't heard anything, we suggest you contact your state tax agency.