1991 Dodge Stealth no spark

when i turn on my car this mornig it started to shake like if it was lossing power, so what i did was that i check the wires to see if they were loose but everything seem ok. Then i remove one of the wires from the coil-ignition with the car runnig to check for a spark and it had spark and then i check the others and from one of the coils there is no spark at all. i replace that coil-ignition and there is still no spark whats up with that, what can the problem be??