My Craftsman 42 " mower's blades will not engage?

"Craftsman 17.5hp 42" LT1000 Lawn Tractor. I changed the deck belt and the first time I used it the blades engaged fine. The second time I cut the grass, it took about 30 seconds for the blades to engage. Now the blades will not engage at all. I checked to see if any springs or parts were missing or bad. I do not see any. The cable moves the pulley and puts it in the engage position, but nothing happens. Also the new belt seems to loose to me. I don't know if it is suppose to be that way or not. I bought the right belt from Sears."