MARK WHITEHOUSE says:
In some large training kennels if a dog shows little promise they can get left behind at the back and become almost switched off , and therefore lose out on very valuable training time.
However, when taken out of this environment and given one to one training and attention as your spaniel has, it is like being given a second lease of life.
Try restricting the area in which she exercises and drill back into her the basics: sit, stay and recall. If this does not seem to be working then I feel the only way forward with any spaniel that you have no confidence with is to seek advice from a professional trainer.
Book a one-to-one training course where you and your dog can both be educated at the same time.
With the problems you have mentioned, chasing of game, not returning with retrieves and becoming very dominating, you need to be very firm, and at the age of 15 months the sooner you seek professional tuition the quicker you and your dog will see the results.