Return Line blocked

[Bkirrane]

Last fall I winterized my in ground pool myself for the first time. I used an air compressor to blow out the lines,

Yesterday I opened it. Everything when well initially. When I began the pump antifreeze and air bubbles shot out of the two return lines. Everything was going ok for about 5 minutes when one of the return lines stopped working. It was only discharging a few bubbles from time to time. Do I have an air blockage in this return line? How do I fix it.

Thank you, Bill