Hey dancing water,
I believe in my heart your mom is talking to family members that have passed and they are helping her to prepare if not to reassure both of you & your mom that things will be ok. I've read some where that in many ways when a loved one has Dementia they are really between worlds(physical world and spirit worlds). It makes sense when you think about it. On one had she is having visitors from the other side who have crossed over(which is probably another reason why you aren't able to feel them, cause they already have crossed and aren't connected to this world but have come back for your moms sake). On another hand you are aware of whats going on so in a way they aren't afraid to visit your mom when you are in the room. Remember many ppl over look this behavior thinking it just has to do with the illness. Have you asked your mom where the loved one is sitting when she has a unseen visitor? Ask questions, if you aren't already. you might find out some really cool things and evidence that your mom is in fact having family members that have come back to visit
Hope my thoughts give you some help. Wishing all the best for you and your mom.