Driveshaft sliding out of transmission coupler

[Zac Daddy]

Redhawk 32 walk behind is the mower. The last few cuts I have had to stop multiple times to tap my driveshaft back into the transmission. It keeps sliding out of the coupler. There is a snap ring that is supposed to hold it in place but the snap ring keeps coming out of its groove and just slides up the shaft as the shaft pulls out. I have adjusted the belt to take out any slack thinking it would help to keep the belt aligned in the guide roller just below the pulley on the driveshaft, but it didn't work. It just started happening one day. I hadn't made any adjustments or changed the setup at all. The snap ring won't slide back into place without some help from a flathead and hammer so I think the snap ring is still good. I'm going to replace it though. Because the only other thing I can think of is replacing the belt and that's not cheap. if anyone has any input I'd appreciate it.