If you mean by "debug" that the highlighting is switched on (Data flow shown graphically as it happens) then your problem is most likely timing.
LabVIEW operates many operations in parallel (or almost at least) unless there's data dependency between functions. If one function relies on another, but they aren't linked by data flow, it can happen that they're executed in the wrong order without highlighting. WITH highlighting the execution is much slower, and the actual order of execution may be different. Try linking all functions which are interconnected with data (The error clusters are great for this). Shane.
