Version 3 signatures had a fixed memory limit of 64K per signature engine. If you attempted to enable more signatures that the available memory, the engine would fail to build and the signatures wouldn't load.
Check your log when you enable IPS on your first interface, it should give you plenty of information.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...