Excel formula does not bring back results in all rows

I have an excel formula that when I copy it down works in some rows but not others. The formula is =CHOOSE(IF(AND(ISBLANK(P:P),ISBLANK(BS:BS)),3,IF(ISBLANK(P:P),2,1)),P:P,BS:BS,"Unknown") The data is blank in column P:P, so it should bring back the data in column BS:BS which it does on some rows but not others. The columns are formatted as General.The data in column BS is from the same source, same format. If I copy data from the working row to the non-working row, I still get blanks. What am I missing?? Thanks!

Thanks Special-K!! That was quick, I cleared column P in those rows and everything works!! Thank you.
Joanne