First guess would be the Instant Anti Reverse dog isn't located correctly - it needs one of the tiny copper leaves on either side of the driveshaft cog.
It's dead easy to reassemble the reel with the IAR dog sitting directly above the driveshaft cog and not engaged at all.
I can post a picture of what it should look like and how it locates if you need it.
I reassembled an Abu TGC6000 recently with the Instant Anti Reverse dog correctly seated,
but installed backwards
The reel wouldn't forward wind at all, but it worked perfectly if you wound it backwards
That would be my first guess - if you don't know what it should look like give me a shout and I'll take a couple of pictures for your reference