That seems about right. You can't get a ton of DNA from 120 ul of whole blood. WBCs are the only ones that contain nucleic acids.

from that amount of blood I would not expect to get much more than that

I too have had troubles getting good reproducible DNA samples from mouse blood. I have looked at urine, and feces too with like difficulties.

The best, for me has been from tissue, ex.. tail clipping, or ear hole punch tissue.

Best Wishes

Agreed

It Is much easier to extract DNA from mouse tail or ear snip

This in my experience having spent 5 years doing it is robust, reliable and can be performed by crude heat extraction of tissue with alkaline hydrolysis or by SDS NaCl prot k digestion for a better yield