I have a gas fire place. The pilot light stays on and the main burner comes on, but the flame is not even close to what it was when it was first put in. What is wrong?
Sounds to me like a simple case of a blocked jet. Depending on your competencies with gas appliances either get someone to service it or you can do it yourself.
For the latter, locate and remove the jet for that burner, normally located closer to the know than the burner itself. Using a proper jet cleaning wire clean the jet by pushing the wire into the jet from the outlet end, so that any crud is forced backwards, rather than through the jet.
While you have it apart, check the inside of the burner pipe and remove any scale that has built up, which could come loose and reclog your jet.
Of course if you can get hold of a replacement jet you may find it worthwhile simply replacing the clogged one.