Check your symptoms and get the most likely fixes, in order.
A pool skimmer depends on the correct water level, open suction lines, clean baskets, a working weir door, and a sealed pump system. When one of these pieces is wrong, surface debris may stop moving toward the skimmer or air may enter the circulation system.
Weak pull is often caused by a clogged basket, dirty filter, closed or poorly balanced valves, low pump speed, or debris near the pump impeller. Start with the simple checks before assuming the underground line is blocked.
Air entering the suction side can reduce skimmer performance and cause bubbles at the returns. Common sources include low water, a damaged pump-lid O-ring, loose drain plugs, leaking unions, or a suction-line leak.
When one skimmer is strong and another is weak, the valves may be unbalanced or one line may be partially clogged. Adjusting the valve position can help, but a completely inactive line may need professional testing.
Return direction has a major effect on skimming. Aim the returns to create a gentle circular surface pattern that pushes debris toward the skimmer instead of trapping it in corners.