Dryer not heating

I have a Whirlpool gas dryer that is blowing cold air and I do some of the repairs myself but 7 months ago I had someone come in and he replaced the coil and now it is doing the same thng and of course he only warranteed his parts for six months. Can it be the same part again? and if so is that a common problem for these coils to be defective? If not, what other parts may be the problem. Runs good just blows cold air won’t ignite.