A dull, buzzing, "I can't feel much anymore" feeling during vibration usually means your nerve endings have got used to a signal that has not changed for a while. Strong vibration on sensitive skin before you are aroused makes this happen faster.
Turning it up usually makes it worse. What tends to help is change: move the toy somewhere else, switch patterns, lower the setting, or take it away for a minute and come back. The feeling usually returns after a rest.
If numbness, pain or swelling lasts after you stop, stop using the toy and talk to a doctor.