Why Do I Get Bubbles Coming Into My Pool From The Return Line?

The strainer cover could be loose or the gasket is damaged.  Check and replace the cover or gasket if necessary.

The pool water level could also be low, allowing air to mix with water through the skimmer.

The skimmer weir, sometimes called the flapper, may be stuck in the up position, allowing air to mix with water in the suction line.

There can be a leak at any connection in the suction piping or a leak inside any suction side valve at the stem “O” ring.

There could also be a leak in the underground piping caused by a loose joint etc.