A VAST wrapper is a VAST response that points to another VAST response instead of a playable video. Verification vendors, DSPs and exchanges each add their own wrapper, so a single impression can resolve through a chain of them before it reaches the inline VAST that actually contains a MediaFile.
There are two distinct failure modes, and telling them apart is the whole job. The first is depth: the chain has more wrappers than the player allows (often three to five), so playback stops with VAST error 302 before the media is ever requested. The second is latency: the depth is fine, but the total resolve time exceeds the player's timeout, so the media arrives after the slot has given up.
To diagnose it, trace the wrapper chain hop by hop and check each hop's status and response time, then confirm the final inline response has a MediaFile the player supports. The fix depends on the mode: reduce or pin the wrapper chain for a depth problem, or address the slow hop (or raise a reasonable timeout) for a latency problem.