supplier bill variances

I have been posting the Supplier Bill variances to clear the accruals account down and I have also been closing down old Purchase orders. Most of them have worked perfectly but I have a few postings that have not netted out to zero and I am struggling to understand why, when all the others have. My understanding is that the Item receipt plus the purchase invoices and the supplier Bill variance journal should all net down to zero but if it doesn't is it because I need to make another posting or is there something wrong that I need to