Their empire was too big, they couldn't control it. Soon enemies began to chip away at the empire little by little, until it was deemed the "sick man of Europe". It took a big hit from the Safavids, which basically was one of the tipping points. They also had bad/corrupt government officials,and regional officials retained revenue for themselves. The Sultans became incompetent, the Jannisaries refused to upgrade, and they eventually fell behind the Europeans and failed to expand.