My Freon is fine, my heater works and the compressor engages. Is it the expansion valve or is there many other possibilities of what it could be?

Check your high and low side pressures. The high side pressure should be the ambient temperature plus 80-100psi, and your low side pressure should be about 25psi. That will help you to determine where the problem is.

Not really enough info to give you a definitive answer. Even though the compressor comes on, the compressor may not be building up enough pressure. It could be the expansion valve but, there could be a restriction (metal particles) in the system that prevent the proper flow of refrigerant. The car really needs to checked by a professional.