Most probably you have a difference in the conc. there (you used different solutions with diff. conc. of Acrylamide/Bis... or didn't mix very well after preparation). Second issue could be that your maschine is not working so properly.

It seems that there may be a leak in the central reservoir (upper buffer vessel). If the buffer in the central reservoir level falls below the inner glass plate, the circuit no longer runs through your gel.

After the assembly, fill the central reservoir only and then wait at least 5 mins. If no leaks (no drips), all is well. If the buffer is leaking, I take it all apart and then reassemble it.

(Alternative: If the apparatus is old, it can cause the leak. You can prepare 1L of running buffer and filled all the tank so even if the buffer level at casket leaked that does not affect. )

Clean well your glasses before pouring your gel and make sure everything is well mixed before pouring.

As mentioned above, it could be a leak in your inner chamber (one where you directly place your SDS plate into). However, from experience, it doesn't quite look like a leak to me as leakage usually makes the dye front curve up a lot more but maybe it's a slow leak that you should be taking a closer look on.

Also to add on to previous comments, here's how I make sure that I have enough running buffer in my chambers. After filling the inner chamber halfway outside of the large chamber to check for leaks, I place it into the larger chamber and top it off with running buffer in there to make sure that I have plenty of running buffer in the inner chamber. Then I fill the rest of the larger chamber with sufficient amount of running buffer, depending on how many gels you're running.