This is a bug in 6.1.6.11834 build 1642 (but currently being the latest workbench, you can't yet update to fix). I also have this problem...
The first time any EER Diagram is opened or created, tables can happily be click-and-dragged about, but closing and immediately reopening the diagram with tables tends to prevent click and drag operations. To fix try opening and closing additional EER diagrams and the tables may come unstuck, but a definite fix it to close the model first, then restart workbench.
Most simple repro: New model, new EER Diagram, create a new empty table, close the diagram, reopen that diagram, table can't be moved.
New: I reported this bug and have been notified it will be fixed in the 6.2.2 release