Suction hose in my watering system getting air. What can be the problem?
Leak at a fitting.

Leak from cracks in the body of the hose - common for aged plastic hose exposed to sunlight, for instance. Or hose which has been chewed by animals.

Sucking water fast enough to either create a vortex (whirlpool) in the supply that brings air to the hose-end below the surface, or simply sucking the level of the supply down below the hose-end.