You could ask another dentist for their opinion, but it is also worth trying to ask your present dentist why they don’t feel a bone graft is necessary and why you are having these problems. When an angled dental implant is placed, it is often inserted towards the front of the mouth as this is where the bone tends to be thicker and stronger, so it might be that a bone graft is unecessary. Talk to your dentist to see if they can help your current denture fit more comfortably and securely.