Fan not kicking in

[JimT]

Assuming you have an air driven clutch that locks up in low or absent air pressure and the solenoid shuts off air to the clutch when the engine is off, it sounds like clutch pads/friction material is worn out. Depending on the type and brand of clutch the friction pads can be replaced.

Some helpful things to pass along would be make/model/year of truck. make/model of fan clutch if known.

If the clutch material has worn out that fast (2 months) there's probably a reason. Maybe something (A/C?) was causing the fan to kick on too often? Maybe there's an air leak between the solenoid and the fan hub or within the hub/clutch, causing partial clutch engagement. Maybe the last clutch wasn't installed properly leading to air leak and ultimate failure? Maybe the air solenoid itself isn't opening fully, causing lower pressure to the clutch causing partial engagement? Assuming you have one (as you never answered above), you should probably test or replace the air solenoid, in addition to checking for air leaks, replacing the friction material on the fan clutch and checking to make sure you A/C system is properly charged and not otherwise calling for frequent high fan engagement.

Anyway, without additional information I'm just throwing out guesses. There's more then one type of fan clutch out there, it's not directly linked to the engine itself.