Every pepper I try grows well.. there are times I get one that acts up but grows out of it. So far I've grown: Indian Carbon Ghost, Chocolate Ghost, Red Hab, Chocolate Primo, Black Pearl, Lava Scorpion, Chocolate Bhutlah, and Naglah. Sometimes, I get bad seeds and they wont start at all.
Plants uptake Sulphur.. so you might be putting the plants into a state of toxicity.. once that happens.. it's hard to get them to bounce back.
Refreshing the soil depends on what style your growing with .. I.E. organic with guanos, worm castings, other forms of scat, and animal / plant parts... Chemical ferts like Miracle gro.. or hydroponic water soluble ferts. Every style requires a different approach.
When a plant drops it's leaves, it means something. What do the leaves look like when they drop? Are they still very full, and green? Or are they shriveled, and dry? Details matter when it comes to what the plant is trying to tell you. How much N-P-K are you giving it? Because, that matter as well.
If you're seeing a 1.5 cm growth in 1 week.. that's not bad... it means it's trying. I wouldn't do anything different just yet. Remember, at this point.. not only is it trying to grow leaves, but also roots at the same time. It has to ration out the energy it accumulated during the day for both.