If you are still unable to play certain discs on the 650BD/651BD/751BD/752BD it may be due to BD-Live content on the disc. In this case, it is worth checking that the Persistent Storage (found in the Device Setup menu) is not full. Persistent storage is the space dedicated for downloading BD Live extras and is 1GB.
Some Blu-ray discs more than others contain large amounts of BD-Live information.
If you have the player's Ethernet connected and BD-Live turned on when trying to play one of these types of disc, upon insertion into the player, the disc will automatically cause the download the BD-Live data onto the player (the amount varies but can be 20-30MB or more) which is required for the disc to play. If the persistent storage is full, or if there is not enough space remaining to download this data, then of course the disc will not play.
By erasing/clearing the BD-Video data, you can clear enough space for these discs to download the data that they require to play.