The IRS says they pay 9 out of 10 refunds within 21 days. That means that one out of every ten, or 10%, take longer than 21 days. Obviously you are among the 1 out of 10 that takes longer. If the IRS didn't say that there was any particular problem, or that they need more information from you, then all you can do is wait. It happens, and it's not all that rare. It's one out of every ten.
If it's any consolation, when they eventually pay your refund they will pay you interest on it, since they took more than 45 days to pay it. Of course the interest is taxable income, just like interest from a bank account. You will have to report it as income next year on your 2019 tax return.
