I use FL Studio mostly for quick brainstorm ideas and I've clicked in a drums sequence and I have an 808 bass along with a punchy kick, both hitting at the very beginning of the pattern. For some reason, the very first hits are really loud, then for the rest of the loop everything is quieter. I've made hundreds of FL Studio beats and I don't believe this has ever happened to me. I always bring up my blank project file that has all my volume settings and master volumes set, so I haven't changed anything in that regard.