Invoice auto increment quirk

[Skinner]

Hi, one of my clients has 3 servers that get invoiced for within a 7 day period, I have auto invoice increment set but each invoice generated is just a follow on from the last, 3214, then 3215 etc, why is it not generating as set in the auto increment? When an invoice is generated for a different client the increment kicks in? Any ideas on this please.

 

Thanks.