This is a common problem with Delta valves.What I have found that works the best is striking the chrome cap in the grooved area of the cap.You need to do this sharp enough to be effective,but light enough so it’s not destructive.I always explain to the customer that this attemp may not always work and the valve may be damaged.The three copper tubes are a weak point and this is where the damage will occur.If you do get the cap off and make the repair I would suggest that you leave the face plate off so you can inspect these tubes after you turn the water on.I do this myself after paying for a cieling.