I have crowns for my front teeth but there is still a gap: What can I do to close it?

Two years ago, I went to go get crowns for my upper front gap. I decided to only work on three teeth, leaving the front right tooth alone. After the procedure I was told that if I didn't want to have a gap, I should do the front right tooth along with the other three but I said no... After the crowning procedure was done, a year later, I decided I want to close the gap.. What procedures can I do to close the gap or will it be hard since I have crowns?