New HVs are expected to take on the full load as those who have qualified for a long while. It's a one year Masters course, partly in university, partly in the workplace, and it's impossible to cover everything, so there are often times she isn't entirely knowledgeable as others may be.
But she acknowledges that and works out a plan to compensate for that, like a second visit, or a phonecall to a colleague, she doesn't just dismiss it or ignore the need to give you answers. Newly qualified, they are meant to have a smaller caseload and only be given the less complicated visits, but it doesn't happen. She is technically qualified, and if they aren't then they should tell you they are a student HV, and usually be shadowed by a qualified HV.
There aren't enough HVs at the moment and they are recruiting for more, so if they often aren't available, it may mean that they are currently having to deal with a large caseload of safeguarding visits, which are generally time consuming. Not returning calls is rude and unprofessional, but there being a delay is often unavoidable.
If you have a HV you don't feel happy with, you could ask for another one, because if there is no rapport there, then obviously it's not good for you and the baby. Give feedback if you feel they aren't doing something right, they'll get more training/disciplined if they are consistently getting it wrong.
I've been uncomfortable with certain HCP and have always been able to ask to see a different person, like when I specified not wanting to see any male social workers.