I think that this should solve it now, I had missed out an important part of the formula

=SUMPRODUCT(COUNTIFS(Table2[User ID],">0",Table2[WC Date],CHOOSE(

**{1;2}**,"=","<"&TODAY()-28),Table2[OCT Call],CHOOSE({1,2},"=","<"&TODAY()-28)))

The way the formula is written it counts in 4 parts

Blank WC date + Blank Oct Call

Blank WC date + Oct Call over 28 days

WC date over 28 days + Blank Oct Call

WC date over 28 days + Oct Call over 28 days

Then adds them together. The incorrect formula, without sumproduct, was only giving you the result from the first part (blank + blank) because there was nothing to add the results together.

edit:- to clarify, the formula will need to be changed in the sheets where you were getting the correct result from the original formula, otherwise any changes to the data may not be counted correctly.

Also, I nearly forgot to mention that I had to change a comma to a semi-colon to make it work correctly, so this will also need changing in the sheets where the formula appears to work. (highlighted in the formula above).