excel nested if and statement - can't see why it won't work

I have these if and statements. They all work. However, when I nest them, they don't but I don't know why.

I have two columns: R and S

S contains a time stamp and I need to evaluate if the reaction times in column R fall between to specific values. E.g. in statement 'A' I'm evaluating if reaction times occurred between timestamps 750812 and 859723. Below these statements are my attempt to nest the statements so that I can just run the code down the column without having to separately check. Kindly let me know if you can spot the problem.

=IF(AND(S2>$AB$5,S2<=$AC$5),1,0)       A
=IF(AND(S27>$AB$6,S27<=$AC$6),2,0)     B
=IF(AND(S45>$AB$7,S45<=$AC$7),3,0)     C
=IF(AND(S69>$AB$8,S69<=$AC$8),4,0)     D
=IF(AND(S85>$AB$9,S85<=$AC$9),5,0)     E

Nested:

=IF(AND(S2>$AB$5,S2<=$AC$5),1,IF(AND(S27>$AB$6,S27<=$AC$6),2,IF(AND(S45>$AB$7,S45<=$AC$7),3,IF(AND(S69>$AB$8,S69<=$AC$8),4,IF(AND(S85>$AB$9,S85<=$AC$9),5,0)))))