I had similar problems landing soldiers on Luzon. (RFB+RSRD), This worked for me.
1. Choose drop off point within 2800 yards of marker.
2. Arrive at drop off point at periscope depth,sub pointed directly towards marker.
3. Stop sub completely (0 knots). The raft goes across the bow and if you're moving forward you'll crash into it.
4. Surface the boat and dialogue box appears. Click on launch raft and watch the fun. If the sea is rough the raft will capsize, but you still get the credit
It seems one has to surface from periscope depth to trigger the dialogue box.