Check to see if the pressure on the reverse clutch pack test port on the valve body is between 105-125 psi at idle and 105-140 psi at 2000 RPMs. The link below shows this on book page 45 and 46.

http://gsps.doosaninfracore.com/Dealer_bulletins/SM/SB2102E01.PDF

If you have good pressure there is a possibility that the piston return spring in the reverse clutch is not made correctly. The spring will compress all the way it is allowed to and not allowing the clutch pack to tighten up. Daewoo sent us new springs that were not made correctly. We ended up having the springs custom made. Daewoo never got back to us admitting there was a problem with there replacement springs.

http://gsps.doosaninfracore.com/Dealer_bulletins/SM/SB2102E01.PDF