Formula sometimes returning correct answer and sometimes returning 0

Hi, after the brilliant help last time I have another query:

I have entered the following nest of formulas:

=IF(('Current Employment'!N2-'Personal details '!O2)/365<1,"",(ROUNDDOWN(IF('Personal details '!N2="",0,IF('Personal details '!O2="",0,('Current Employment'!N2-'Personal details '!O2)/365)),0)))

and then copied and pasted it down a column. It is to compare two days and then return the difference as whole completed years. Sometimes, it returns a correct result and sometimes 0 when the result should be an integer even when the two correct dates are entered in the correct place. It has me totally foxed.

Sorry I cannot attach the sheet as it contains confidential information.

Edit to add example:

I have done a simple

=('Current Employment'!N170-'Personal details '!O170)/365

and it has returned the correct answer of 9.34 but the above formula as

=IF(('Current Employment'!N170-'Personal details '!O170)/365<1,"",(ROUNDDOWN(IF('Personal details '!N170="",0,IF('Personal details '!O170="",0,('Current Employment'!N170-'Personal details '!O170)/365)),0)))

is returning a value of 0