Richard,
There is no simple answer to your question. To help you I need some questions answered. First, is there gas flow at the burner when the unit is first turned to "Gas"? Second, is the igniter arcing before the "Check" light comes on? Then if the answer to both of these questions is yes, what does the flame look like, and is the burner chamber unobstructed?
After these questions are answered, we will be able to narrow the problem down to a specific area, and be able to help you repair the problem.
Gerald