At this time of the year, there is an increase of soil moisture problems that start with browning of leaf edges. This type of problem can be confirmed by checking the soil using the finger method. Insert a finger to a depth of 4" and water only if the soil feels almost dry or dry. While you are at it, recheck the mulch and add more if you have less than three inches. A soil moisture problem however, would not cause curling in older leaves but it would cause browning throughout many of the plant's leaves.

Curling could be caused by a myriad of things: moisture problems on new tender growth, insects (aphids, hydrangea leaf curler, borers, etc), by a fungus or even by some fungicides/insecticides.

So why is your solution not working? Could be that it is a soil moisture problem or it could be a case of the correct diagnosis (insects) but the wrong solution for that type of insect infestation. The problem with insecticides is that you first need to do something that can be darn difficult. And that is identifying the insect correctly so you know that the product will take care of the problem. For example, if worms are causing the problem, you need to apply Bacillus Thuringiensis; if aphids are causing the problem then apply Bonide's Eight; etc etc etc. Once you find a possible causal agent, you can take a sample in a sealed transparent plastic bag to a local nursery or extendion service for identification. So, check a few times during the day and ALSO at night (when many pests come out of hiding) to see if you find any active pests causing damage to the leaves (look at the underside of the leaves too).

By the way, it does not sound like sun damage. Too much sun initially causes the leaves to turn all yellowish or even white-ish.