I'm having problems with the IF function, I hope someone can tell me what's wrong and point me in the right direction.

I'm trying to make a sheet to help with Caravans and License Restrictions, my cells are as follows.

In cell F21 I have the following:

=IF(D12<D21,"OK","B&E Required")

Cell D21 has the following in it:

=IF(B21<1997,"7500", "3500")

I think the reason it's not working is because the cell I'm referencing has the values inputted by another IF function. I have changed cell D21 to D22 and entered the values manually and it works how it should.

Where am I going wrong?

Cheers in advance