I had 13th day 14mm dominant follicle, 4 days later it was 15mm,then it was smaller and i didnt ovulate, what are the causes if have regular periods?

Not clear: I am not sure definitive conclusions can be made from monitoring follicular development over one menstrual cycle. Since you are tracking follicular size I assume you are seeing an infertility specialist otherwise I am not sure why one would be monitoring this. If so then schedule a follow up visit with the specialist to determine the next steps.