If the glass plates were leaking from the bottom end which happens in most of the cases then you could try sealing it by using agar. After agar solidifies then add the separating gel. Also as a routine practice, after assembling your glass plates, you must check the leakage by addition of water.
Make sure that all three plates (2 glass plates and the spacer) are even from bottom (both left and right side). If not press the entire set against even surface and clip it before placing into the chamber. Check for leakage using water after fitting into the chamber.
The type of casting equipment you are using is notorious for leaking. Basically, they wear and don't seal properly anymore.
If the above suggestions don't help, I have managed to get a good seal by placing a thin section of rubber gasket under the plates to get a good tight fit.