Tucking her nose in isn't getting her to work correctly. To get her moving correctly her whole body has to be engaged, not just her head. To avoid getting a 'false' contact, you need to do what the others on here have said.
Encourage your horse to drop their head down and reach into the bit. Keep your rein contact consistent so your horse wants to seek this contact. Push your horse into your hands with your seat and legs and keep her moving forwards with impulsion to encourage the lifting of the back. Pole work, hills, circles, lunging with side reins and longreining all encourage the buildup of the correct muscles in the back and stomach.
Alli flies round the field with her nose to the heavans when she is excited, but she is starting to learn how to drop into an outline now- only backed this spring so baby steps!
It can be fustrating when a horse seems to refuse to drop into an outline and engage the back end, but as the muscles build up it becomes easier and the horse will also learn working properly gets reward.
Also make sure the horse is relaxed and the saddle not nipping. Talk to your horse about whatever pops into your mind. A relaxed horse will work into a contact with the correct riding.