The friend who cannot help today should still feel free to ask you another day.

A single no should not close an account. When someone declines a request, it is easy to file it away and then feel unable to ask them again, or to quietly withdraw your own willingness in return. A friendship starts running on a ledger neither person wanted. Availability changes week by week for reasons that have nothing to do with how much you matter to them. Keeping the door open in both directions is what makes asking possible at all.

If someone said no recently, ask them something else soon and warmly. If you had to decline, say clearly that you hope they will ask again.