We suspect that some of the bills might not have generated due to performance problem while running the batch job. we suggest to change the Batch SP to check for the Previous day Unbilled Items on the next day.
(i.e
when batch job run on 10 Aug, it will generate invoice for unbilled items raised on 4th,5th and 6th Aug
when batch job run on 11 Aug, it will generate invoice for unbilled items raised on 5th,6th and 7th Aug)