Next to that: do you search at very popular systems? (Means, near Shinrartha Dezra, at or near the known stations which sell for very much, etc. )
I am not sure if my observation is right, but i get the impression that even if you go to hotspots there in a PG or even solo, the if other people in other modes cracked the rocks there already, they are not there any more for you. At least i had problems finding crackable rocks at those places, while instead finding the clouds which are left behind when a rock was cracked.
Once i moved a few systems away and found myself a hotspot there, things got better. Mind you, i still sometimes use like 25 limpets before i find what i want. But that's still better than using 96 and finding nothing, which i did when i was in solo at a massively populated place.
As said: i have no hard evidence to support this. It's just how things feel for me at the moment. Perhaps others can provide better data?