Maggie, PX MID doesn't work for all tickers (not real time for sure). To get PX MId just get PX ASK and PX BID and take the average
=(BDH("IBM Equity", "PX ASK", "08/31/2007", "08/31/2007")+BDH("IBM Equity", "PX BID", "08/31/2007", "08/31/2007"))/2
if this doesn't work try to use the...