Are you trying to clear one line at a time? Also, what is your compressor set to? Have you tried to burp the return lines to build pressure?
What I would recommend is to drain below the returns and clear the lines. In addition, the skimmer lines should be cleared at a the same time a wet vac is being used. I just do not have enough information or know your plumbing/equipment pad to determine where you are loosing air.
Close all the return lines and then open one up at a time as air is removing water, then close for a few second sand open up again. With water below the returns, you will not run into any problems.
What I would recommend is to drain below the returns and clear the lines. In addition, the skimmer lines should be cleared at a the same time a wet vac is being used. I just do not have enough information or know your plumbing/equipment pad to determine where you are loosing air.
Close all the return lines and then open one up at a time as air is removing water, then close for a few second sand open up again. With water below the returns, you will not run into any problems.