In a statement they say:
We have results in from 13 of the 14 constituencies in the Mayor of London and London Assembly election.
The final constituency is Brent & Harrow.
Two batches went to storage without some ballot papers being manually entered as required. Manual entry is required when a scanner cannot read a ballot paper – for example if a ballot paper is damaged. It is not an issue with the scanners.
The issue was identified during the verification stage. These two batches are being re-processed. To make this happen as quickly as possible we have separated out into several smaller batches.
We will declare as soon as possible but it is obviously important that every vote is counted.