THIS either that or the spark is so weak that you just think you have spark and it's not strong enough to really do anything
Test the spark using a WIRE core substitute for the coil wire. Crank the engine using the key, and it should generate nice blue spark at least 3/8" long and typically 1/2"
Check your coil wire with an ohmeter. The old "rule of thumb" for resistor wire was less than 1000 ohms per foot of wire means "good." If you have that then it's got to be a cap/ rotor issue somehow.
Please detail the ignition system. What exactly is the distributor you are using, ie points breakerless, what? Stock Mopar? Aftermarket? And what if any electronic ignition are you using?