Why is my English saddle still lose even if it feels so tight on my horse?

Hi I just got an english saddle and it fits me perfect I ride 2-3 different horses and it fits them well, but when It comes to the cinch its terrible. I can put the cinch on and it be tight and I can put 2 fingers in it to make sure Its not too tight. But then when I get on it slides down to the left. I dont know what I can to to make it not so slippery.