Attempting this raid again for some xmog. The first 2 bosses downed fine, when I get to thrall and start the mobs, it shows 14 Invading enemies, wave 1. Nothing spawns. After a few minutes, it says 28 invading enemies and wave 2, these however do spawn. Currently I’m on wave 6 and all these have spawned except the original wave…
I’m waiting for a couple of minutes between waves for the next to spawn.
Edit:
Wave 7, after a few minutes (when wave 8 was due), Thrall says its over, withdraw and it fails. So I cannot continue the raid. He had full health, no NPC died
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I just had exactly the same issue that you described 
The ‘bug’ only occurs if when you have talked to Thrall, “We are with you, Thrall!”,and nobody in your raid is in the Horde Encampment subzone of Hyal Summit, while the enemy counter is still on zero. That is, if you run too fast towards the first wave, and the first wave is not there yet. Then you come back to the Encampment and you find the counter on 14, but no enemies in sight.
You can solve the situation like this:
After Thrall yells: It is over! Withdraw! We have failed! You just exit the Hyjal Summit zone and reset all instances. Re-enter via Caverns of time, though the Horde Encampment Portal next to Tydormu and wait 30 seconds in the Horde Encampment. Thrall shows up again.