Simple but iratating problems

[SALO]

I am trying to preform simple formulas that I frequently use (and by the way have had similar problems at times)which are Sumif and Countif formulas. For examples in column (A) are Stock tick trade times by the seconds over multiple days. It is in the simple time format 9:30:01, 9:30:02, 9:30:03 etc etc Then in column (B) I have the average stock price for that particular second over muliple days. Using sumif and countif I have summing up and then averaging. Anyway the problem is it works great but sometimes I get to the cell after auto filling the formula down, and then all of sudden the formula dont recognize the criteria which always the time (9:30:01, 9:30:02, 9:30:03). Its in the same format of the source sheet and even the whole columns where I auto fill the formulas is all formated the same. Like I said I bring the formula down and works for 30 or 40 rows and then just stops. I fix it by going to the source sheet (range) and then just paste it directly in the criteria cells. But its pain and there must be a reason for this and easier way to correct. Im using excel 2000. Any help will be appreciated.