Or it could be just desensitization of the nerves from the texture of the toy and the regular use. That should fade in time. If I use my Akita several days in a row, I experience some loss of sensation. Not a huge amount but it's noticeable.
I know whatcha mean...been using mine for the last several days, and it doesn't seem as huge as it use to. I think it has to do with 1. Size 2. Texture 3. Lube (this I have noticed, sometime lube can numb the nerves/cover them so they are less responsive. Also, as you stated, repeated us of a toy seems to lessen the feeling of it, I think this has to do with your body becoming use to having something going in there.