Soil acidifiers are not fertilizers and if you haven't fertilized, right after flowering is the time to do that. In fact right after flowering is the time to deadhead, prune as when needed, and leave it alone the rest of year. You're problem could very well be alkaline soil (high pH) but there is no way to know that without a soil test. Read more here: https://homesoiltest.msu.edu/ The lab cultures a soil sample you send and responds with recommendations for exactly what the soil needs for optimum growth of the plant you name. Otherwise you're just guessing by adding amendments without the soil test.
But soil should not be tested for 6 weeks if recently fertilized or amended, so as to not skew the test results. Since you've just added an acidifier, I might fertilize now (with an acid loving plant fertilizer) and then do the soil test in six weeks. From what I can see in the picture, the situation does not appear to be critical, yet, and the fertilizer watered in should help.