Well, it's not falling off, it's sturdy, so I don't think that's the problem.
Not falling off doesn't mean that it is properly installed and secured. You need solid and even downward pressure on the CPU heatspreader to ensure maximum cooling efficiency. I would remove your cooler, clean off any excess thermal paste from your cooler and your CPU using isopropyl alcohol and cotton wool buds or a lint-free cloth. Then reapply your thermal paste sparingly. You don't need too much since the idea is only to fill in the tiny microscopic gaps between the two surfaces. Too much will have an adverse effect. When you're done the paste should be so thinly spread that its almost see-through. Then attach the cooler again ensuring that you have good even pressure and secure it in opposite corners - do one corner and then the opposite corner instead of going around in order. This ensures it is seated correctly. Try this and then post back with your results.