Easily 60 to 70% of the hard drives that are sent for data recovery are suffering from the same fundamental issue. The issue is “bad sectors”. Every hard drive is composed of a vast number of sectors (a sector, usually 512 bytes in size, is the smallest writable unit on a hard drive). It is perfectly normal for a hard drive to lose sectors over its operational lifetime. The manufacturers expect this and in fact build in a pool of spare sectors which are reallocated one-on-one for one basis as bad sectors occur. The problem is of course that this pool of spare sectors is necessarily small.
Once the pool of spares has been used up every new sector which develops is potentially lethal to your stored data, it may even be that if it occurs in a critical region of the drive you may lose access to that drive altogether. This should only happen after years of service but is far from unknown even for relatively new drives. It is typically the case that once that sectors start to develop there can be an avalanche effect and very quickly you have a large number of bad sectors. Running recovery software on a drive with developing bad sectors is an excellent way of turning a few bad sectors into complete meltdown. If you discover your drive is suffering such an issue then you should approach an expert for disk data recovery.