I have a 2003 ford focus my car use to run awesome until I brought it to a Shell service gas station and they replaced my ignition coils and I remember they had struggle to remove one off, however after replacing them my vehicle sometimes run great one day and the next day sometimes slow like it is not getting any power. Could it be that they messed up one of my ignition coil when they were struggling to remove one? Thanks I look forward to any feedback.

 

Since you already paid for the service, there is nothing wrong with going back and letting them know something is still wrong . There is a good possibility they might have damaged the spark plug in the process of removing the difficult coil. As long as they have the equipment to check and confirm their work , no harm in asking them to check their work.If the mileage is up there it might be time to replace the plugs if they haven't already been replaced. Plugs can be cracked with careless installation .