Replaced the contactor on heat pump as it was bad. It worked for several hours and now will not pull in. I have 24 v at contactor when I checked it. What would cause it to quit pulling in? Could a weak transformer cuasing this or what am I missing?

If you hae tested the voltage on the two poles of the contactor then you have to have a problem with the contactor. The one I suspect is that the low voltage coil on the contactor has an open wire. One way to check this is to turn off all power to the unit, pull one wire off the low voltage spade clips, make sure it is not touching anything, turn the power back on and turn on the thermostat to send power to9 the contactor. Use your voltage meter from the lose wire to the other wire on the contactor, you should get 24 volts, if you do the touch the lose wire with one meter probe and then touch the spade clip where you took off the wire with the other probe. If you do not get upwards to 24 volts then the coil is open and will need to be replaced.
Now if it is bad I do not know what might be causing it to open, it could be high voltage someway. Do you know how to check the amperage on low voltage? If not let me know and I will tell you how and if you have the right type meter.

Just a little simpler to test contactor coil. Remove all power to unit, stat and disconnect. Remove a wire from either side of the coil and test for Ohms, 1-1K ohms its good, open is bad.

Never assume that a contactor is bad. If you don't have 24 volts to the contactor, then you must try to find out where it stopped. Is it coming back to the unit ,from the thermostat? It would read upward of 24 v from common to y. y is the call for cooling. Heating also on heat pumps.
Many units have devices like pressure switches that 'interrupt' the 24v call for the contactor to pull in in order to preserve proper function.
The most common is a loss of charge switch on the liquid line. It is normally closed but when the pressure of the freon in the liquid line drops below a certain set point, the pressure switch will open.
You can wire past the switch (only temporarily) to see if the contactor was just' held off.'
If it is this switch holding it off, you have worse problems than a faulty contactor