You are likely correct - the IRS has been challenged in keeping up with changes in the tax law and processing returns. This is very likely the reason you are still waiting on a federal refund but have already gotten your state refund.
According to the Washington Post (Mar 12, 2021):
"Nearly 7 million tax filers are in limbo and facing substantial delays in getting refunds so far this tax filing season, as the Internal Revenue Service struggles to keep up with the demands of issuing stimulus checks and implementing myriad tax code changes from coronavirus relief packages, including the one President Biden signed this week.
There are 6.7 million returns that have not yet been processed, more than three times the number in the same period last year, when fewer than 2 million returns faced delayed processing, IRS data shows."
The IRS Where’s My Refund? site is still the best first place to go for information.
