There are small but significant variations in diameter between various manufacturers. As well, because Magic-Flight has had to switch battery suppliers, we have also had to adjust the tension of the battery contact coil accordingly. The result is that it is difficult to know which batteries will fit snugly in your particular Launch Box. Batteries from some manufacturers will feel a little loose. This might be annoying but it doesn't interfere with use of the MFLB.
To operate properly, the battery needs only to contact the battery contact coil; a snug fit isn't necessary. To achieve this, some users find it helpful to twist the battery slightly after insertion. You can also look into the battery insertion hole while holding the end of the Launch Box at right angles and see where the coil intrudes slightly into the opening. When the battery is inserted, you can gently pull the battery in the correct direction to make the best contact.