The "flat" term you use makes me think about low contrast, that is not the same thing as incomplete development, but it can be lack of agitation or too short a developing time. How is the shadow density instead? I would use that to judge the amount of development, provided the negative shadows are correctly exposed. On the question whether 100mL of developer is enough or not, I have used Microphen 1+3 75mL developer and 225 mL of water (300mL total volume) with perfect results and no issues at all with incomplete development and D76 1+1 150mL developer+150mL of water with perfect results as well.