I have a very old wall heating unit , the problem is when I turn the thermostat to on it takes anywhere from 5 minutes to 30 minutes to come on and sometimes it doesn't even come on-ive replaced the old thermostat it is a 2 wire system and now I can't get it to come on- any help will do

Most likely your wall furnace is a millivolt system, which requires a special millivolt compatible thermostat. Even with the correct stat, if the thermopile is weak, the unit will not operate correctly.

Make sure you have the correct thermostat for unit, and if unit doesn't fire, change the thermopile. You can check the thermopile, but it takes a certain meter to do so.