[Darryl Ramm]

What sample rate are you operating at and how many total mono equivalent tracks? Are additional voices being used up for plugin processing? You also need to count outputs. Best to work this out yourself and see where you are vs. the system I/O limits and (what is more likely the issue) the voice limits. Pro Tools 10 reference guide has explanations of all this, including a table of voice limits.

This likely highlights some of the pain of old TDM gear, for what is likely no benefit here (no need for low latency monitoring if this is all a live recording) you could avoid these low voice limits with a Pro Tools native system (or just about any other DAW) with CoreAudio or ASIO interfaces and a modern well set up computer (just a laptop in many cases).