Very cold low side return pipe but AC is not blowing icy cold as it should and AC clutch cycles frequently? What would cause it?

YOu hjave TWO sennsors here! One is LOW pressure an one is HIGH pressure! it TRIES tokeep the temop of the FREON in a certain RANGE! If eitehr sensors is BAD or the freon goes OUT of range it wil CONSTANTLYL RECYCLE and go ON and OFFF over an over again! get a REAL AC servie with a MACHINE tht REMOVES al teh OLD freon and cretes a VACUUM and THEN re injects the CLEANED FREON with PAG oil to the RIGHT AMOUNTS! LESS than $120 most times though INFLATION is RAMPANT! >>>LIST CAR TYPE and YEAR and MODEL< too! 

You need to add more 134a. When the AC compressor is cycling too fast the refrigerant is low.

Rapid cycling indicates low evaporator pressure. Most common cause is low refrigerant.

The most common cause is low charge which can be and SHOULD be tested FIRST.

Year make and model.  Check if air flow out of the vents is normal.  Fix the air flow if it is not.

With a normal air flow, connect a manifold gauge and check low side and high side pressures.  If both pressures are low, the system is undercharged.  If low side is very low and high side is low, TXV could be bad.  If both pressures are normal,  there is air or moisture in the system.

Those things are a matter of opinion. Actual temperatures and pressures are the key to a diagnosis. The condensor is often overlooked, a high pressure switch will cycle the clutch due to say restricted condensor airflow or an inoperative fan. If the pressure is normal and the charge is correct then there could be trouble like a blend door stuck partially to the hot side.