My shower doesn't have any cold mixing in...?

For a while it would be cold till a point, then once it turned warm, it would continue its temp increase until it's all hot, and then it won't go back cold at all. Turning it on and off several times would eventually cause a loud *thunk* and the cold water would return, in the same fashion. Now however, I can't get it to do it's thunking business, and all I get is hot water. I turned off the water and took the regulator apart, and everything seemingly checked out. Put it back together and no dice. I thought it would be the sliding regulator but that doesn't make sense with the *thunking* sound. Should I just have my landlord call the plumber again? Last time he came he told us "It's fine you just have to wait 14 seconds for it to change over." Bull.