Jake1222,

If your cruise control works fine above 50mph than forget all the good and correct advice above. It happened to my car too.
When I acquired it about 15 months ago, the CC didn't work under 50 mph. The previous owner never used the Tempomat at lower speeds! As I started to use it on every speed, slowly it began to respond on lower speeds. After a few months it worked and works still as it should. For my W 124 it starts working at 40 kmh ( 25 mph ? ) and cuts out under 40 kmh.
I can easely feel it : if there is, say 70 mph in the memory and I engage it at say 10 mph, nothing happens. When I continue to accelerate slowly I can feel at 25 mph the "kick" when Tempomat takes over control. Mine is a 300TD turbo and I assure you : you feel it when she accelerating !
So with a little luck your problem will solve itself. It only will cost you a little patience!

Danny