Joanie, is it a gas oven? And does it have an electrical ignitor? (not a pilot light)
After months of talking nicely to the oven and praying that it would behave today (sometimes it lit right up, sometimes it took a while, sometimes a long while) the THIRD repairman simply replaced the ignitor. They get tired, (the ignitors, not the repairmen) so that they're heating up but not quite hot enough. It's a simple task if you can get the piece.