Could definitely be a pH issue. Also what's your watering schedule like? Is your medium as dry as it looks? What's your drainage on that cup like, does water flow easily from the bottom when you water?
But for real, knowing your pH is a big deal, the water you use and what the run off from said drainage let you have an idea of what the plant is experiencing. If you know what pH the water is you also have an idea of what it's going to do when it hits the medium.