Invisible blocks in multiplayer

Every time i play multi player with a friend or something the blocks places and the blocks are altered to something else. If there is a mountain in the host map, the person who is with me playing multiplayer cant see it. I tried everything to try to solve it like reinstalling and stuff.And if there is a mountain in the host game, and the person who is not hosting cant see it, they just walk through it and lose half a heart of damage for each second. Oh yeah its survival by the way. And it only happens to my main world. If i make a new map it is solved but i dont want to make a new one.