To move them back, go to LW's Library window, click thearrow next to Incomplete at bottom to open it, then select all the files & press Resume button. That will force them all back.

As for preview problems I am not sure. There are some video types that don't seem to preview or preview well for some odd reason. Best thing to do is go to incomplete folder in Library window, find your incomplete file & press Launch. If it doesn't open then it's a problem with your player or the file itself. (a) Make sure you have up to date CODECs. (b) Consider using VLC as a preview player. (c) Sometimes files don't download sequentially from start to finish but may download the first few bits or bytes then download a section in the middle or near the end etc., so the player cannot find the starting point or the needed bytes that tell the player what file it is & where to find the beginning point have not yet been downloaded.