I've had that problem in the past. Even though it has undergone improvements, I try to avoid sleep and hibernation now. I think your problem is definitely software and depends on how a specific application works with hibernation. I also think the more apps you have open, the greater the chance of a failure to resume from sleep or hibernation. Applications and drivers can also leak memory, not returning memory resources they have used when they complete operations or are shut down. I think the longer your system uptime, the greater chance of failure to resume from hibernation.