Hoping Kal or Dr.Muramasa are going to address this. Dr. Frost dialog seems to freeze up after pregnancy and you end up stuck in a loop where you cannot get to the experiments menu to claim pills anymore. The only way around this would be ensure all pills are claimed before she gives birth. If this is intended it needs to be made more clear or if it is a bug it needs to be fixed.
In regards to how to obtain futa pills I believe the intended workflow is:
In the Haunted House, you are supposed to hide (click text or hit spacebar) when she gets close. Though when she catches you get a couple of bonus CGs so worth getting caught at least once. Keep the light on Astrid until the bar fills up. You need to do this 3 times and after this the Vampire will show up when you return to your room. NOTE - You do not actually get Vampire Spit here I think it is a visual bug, you get it on the 4th time of completing the Haunted House after doing anal with Astrid.
As with mentioned above this is when you give Dr. Frost the Vampire Spit (But if you are locked out in the after birth loop this cannot happen)
You then use them in the Studio > Porn
Then there is the visual display bug with Anna CGs and the whole "Nathan" thing 😅 as within the OG comment