If you are using one of the neon testers, it could be a couple of things. Don't always trust them, even though I have one in every tool box. They only tell that the coil is generating electricity, not how stroung it is. First, you may have a bad plug or one where the gap is too wide. Second, your coil may be going bad, have a bad ground or improper air gap. If you have or can get a tester with an adjustable gap, set it at about 1/4" and see if you have spark. A rule of thumb which I learned too long ago is, "if the spark will jump a 1/4" gap while not under compression" coil is good.
