The ignition wire should have been part of the set. What seems to happen is changing some of the ignition system has an overall effect, since they aged together they have kinda learned to live with each other. When I changed wires, I had some starting trouble. Luckily I have another volvo with the same coil and did a swap just to see, and it cured the problem.

How old is the coil? It may need to be replaced too. It's a good idea to have the whole ignition system at the same newness level, like changing both rad hoses, or both headlamp bulbs. Then you have a better feeling that you're fine for a long while.