Understanding Society User Support

Good afternoon,

I have a question regarding the weight variable xrwtuk1 in the BHPS. For my analysis I have selected cases who have a valid response to my two main vairables (economic status and GHQ-12), and then had a look at the weights. Within the cross-sectional weight variable for the UK (xrwtuk1) I have around 2.5% of cases which are '0' weighted. All of the cases I have selected are full interviews, and the '0' weighted cases have a range of sample origins (67% are OSMs). I have read the weighting section of the user manual but I cannot find a reason that matches the data. Please could you advise me on what may be the cause of the '0' weight?

Kind regards,
David Bayliss