Vlookup Only Working Half Way Down Spreadsheet?

I have a spreadsheet with members details in and I've added a few columns showing:

Col Q: Paid 2008 (blank if No, 1 if Yes)
Col R: Show Paid Yes or No (shows a Yes or a No)
Col S: Date Joined (only entered if new and joined during 2008)
Col Z: Subs owing from 2008 (if existing members and owe last yr's subs)
Col AA: Subs to Pay in 2009 (a standard £35 or pro rata if joined during 2008)
Col AB: Pro Rata (if they joined other than at the beginning of 2008)
Col AC: EA Sub? (based on a yes/no column, to add £5 if yes)
Col AD: Total to Pay for 2009

So the formulae (or data entered) for the above columns are as follows:

Col Q: Paid 2008 - blank if no, 1 if Yes
Col R: Show Paid Yes or No - =IF(Q2=1,"Yes","No")
Col S: Date Joined - a date entered in format d/m/yy
Col Z: Subs owing from 2008 - =IF(R2="no", 35,0)....I want a default 35 in here
Col AA: Subs to Pay in 2009 - =IF(S2="",35,"Pro Rata")....says that if there is no date which indicates they are an existing member, they will pay the standard 35, otherwise they'll be paying a pro rata fee...................

Problem:
It all works fine until a certain row (27 actually) - then I get a VALUE! error which points to the Date Joined field. Now I've tested that value against the vlookup table and it returns the correct data, so why doesn't it do it in my spreadsheet????? I've tried changing the formats on the cells; clearing all data; entering a different date, but from that row in the spreadsheet down to the last row, it simply won't work! I've looked at each formula in each cell on the rows where it is working and the rows where it isn't and I can't see anything different.

What else can I look for? It is SO frustrating as it does what I need it to to do but only for a third of the spreadsheet!!!!