In my own experiences spots on old leaves (spreading quickly to middle leaves) mean your PH is off. Repotting should help if the new soil has a better ph. Consider things like (1) how much solid residue is in your water you feed them and how will it alter the PH over time with multiple waterings. Salts will steadily alter the PH in the soil if they build up (2) how your nutrients, if you are using any, effect the ph of the water once added. For example my tap water is really alkaline, but when I've added nutes it becomes quite acidic. (3) What PH your soil is to start with, this is important. Test the run-off water of some fresh soil as this is your starting point. (4) lastly by testing the run-off water of the plants after you water/feed them you can see where you are at in terms of PH. A simple bottle of PH Up and PH down can be used sparingly over multiple waterings to bring the PH to wherever you need it (roughly speaking between 6and7). A simple PH tester (the chemical one I use) is cheap as chips.