I normally call the thermal expansion valve a TXV. They come in two versions. External equallized and internal equalized. External has a 1/4" line that goes towards the outlet of the evaporator, and ties into that line with a 1/4" flare fitting. Do you have that kind? You will be able to install a 1/4" flare tee, and check the pressures there if you do, and that will help in figuring this system out. You will be able to start the system, confirm that you have about 15 PSI near the compressor, then check to see what the pressure is at the outlet of the evaporator. If there is a pressure drop in this line, you will know what part to replace.
Also there might be a huge pressure drop in the line leaving the condenser (at 150 - 200 PSI in your case) to the TXV (should be very close to the pressure at the outlet of the compressor - might be 20 PSI less). If there is a huge pressure loss, such as a bad freon drier, this will cause the freon to stop flowing. The freon drier will absorb moisture, and needed to be replaced when it has been 'open' and not charged with freon for more than a few hours. The drier being plugged will cause the system to act like it is.
Also if you had a internal equalized TXV and installed a external equalized TXV (or the reverse) then the system will not work right. So if the old one had 3 lines and the new one does as well, you are in good shape. If the old one had inlet and only 1 outlet, the replacement is the same, you are in good shape. But Don't change from one to the other without dealing with the extra freon line.
You might have a plugged TXV. It is possible that some junk in the condenser has migrated toward the TXV. IF you are going to the trouble to open the system, install a tee in the TXV external line connection and have to recover the freon anyway, you can blow nitrogen from the compressor fitting high side to the TXV. Loosen the TXV input line, and make sure that the nitrogen flows freely from one end to the other, all the way through the condenser. IF there is back pressure, there might be a freon drier in the way, that is plugged, and that might solve your problem.
How about remove the TXV bulb clam, and warm it. This will open the TXV all the way, and should increase the pressure in the low side a lot. IS the freon line leaving the evaporator already cold? My guess is that at 15 PSI, if you are only getting minimal cooling on the coil, that the air is warming what little freon is in there to about 65F and it will come out very warm. The TXV should be opening fully and the pressure should increase to say 30 PSI. This should flood the evaporator with freon, and cool it, when the outlet is cold enough, then the TXV should respond and close the valve a little bit.
You might also need to put a pressure port at the inlet to the TXV, so that you can determine the freon pressure into the TXV. This should be close to what the compressor output pressure is. So you are getting 150 PSI at the compressor outlet, it should be within 10 PSI at the TXV input. My guess is there is a freon drier inline between the condenser someplace that is blocking your freon flow, and your TXV inlet pressure is much less than the compressor outlet pressure. So my guess is TXV inlet pressure is going to measure only about 40 PSI, while it should be 200 PSI if that is the compressor output pressure.
Or it is possible that if you had a internal equalized TXV, and installed a external equalized TXV, that is the problem.