If they did not use a/c delco/delphi parts, anything is possible when saving a few bucks.
The plastic could be loose fitting, or not notched out correctly.
How long ago did they work on it?
Do you have your repair order receipt?
See if the tech made further recommendations.
Since you did not identify the make model, year, etc.
Engine, trans etc.
Mere speculation is all that will be available.
Please update your question with enough data that someone can help.
You may just have your wires crossed.
pull the cap off and check the rotor to make sure it,s sitting down flat and check to make sure the cap is sitting straight !
Sounds like they messed up the timing somehow.
So I had my truck at pep boys to get a diagnostic cause my truck was acting up. So I was told it was the distributor cap and to change the rotor as well. So I did and now it's worst
