Blowing Out Lines - Air Bubbles After Plug Inserted

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I was closing my inground pool with vinyl liner today. I had the compressor hooked up and air bubbling out the jets. As I put in the winter rubber stopper plugs and turned off the air, I still noticed a little bubbling, even after inserting the rubber plugs and tightening them. I couldn't quite tell where. It doesn't seem to be coming from the plug. Possibly from the flange? Does that make any sense? Maybe it is just the plugs not seating. I'm going to order and try the screw in type. But any hints greatly appreciated. Thanks.