Long term storage of agarose-ethidium bromide gels that have already been cast
To increase storage life: after gel solidifies, dampen it with running buffer. Wrap the gel in polyvinyl chloride. Place in plastic container with a lid. Store in fridge in dark. It will last for a year as long as you re-dampen it with buffer each time you access it. Don't keep it submerged in buffer as the etbr will diffuse out. this is probably the cause of your problem. Notice when you order precast gels (either agarose or acrylamide) they are sealed and only a small vol of buffer is added to keep it hydrated. Additionally, the etbr and EDTA in the buffer helps prevent most mold growth although I have seen black mold grow a few times at around a year.

You can always spike the buffer with etbr adding it directly to the electrophoresis chamber before you run the gel (etbr doesn't even need to even be in the gel for it to work) although I find you get a little more sensitivity with the etbr in the gel. If you spike it be sure to mix and let it sit for about 20 min for the etbr to diffuse.

If you are trying to cut down on cost you are probably using a higher grade agarose than you really need for the application your doing so I would take a look at that

If you end up needing to store gels for rna work, add plain old Clorox bleach to a final concentration of 1% to your TAE gel just after microwaving. It sounds crazy but it works extremely well. You will completely inhibit RNAse and your rna bands will look nice!!!. The bleach also helps  with storage of the gel.