If your horse has a tendency to hollow in downwards transitions make them very progressive by shortening the stride shorter and shorter by multiple half halts, whilst keeping your legs on to keep the power. Keep shortening and bringing back as far as possible without allowing the horse to break, before moving forward back to a working pace and repeat each time asking for a bit more ‘sit’ behind so engaging the hind legs. You won’t necessarily be achieving a true collected walk/trot/canter but it will be a solid start towards it. You will feel like you are getting slower and slower but you should still feel the power coming from behind.
Once your horse is working responsively forwards and back, when at the most collected if soft through the frame, instead of pushing forward to working pace, ask for the downward transition but keeping your legs on to ensure it is a forward transition. You should quickly find your transitions become more balanced and forward and lose the hollowness. You can then work on making the transitions more direct, ask for the collection over a shorter number of strides until eventually you should ‘just’ need a good half halt for a smooth transition. You should start with the walk halt transition, then trot walk, then canter trot and finally it should enable you to perform a direct canter walk transition.
Asking for this level of collected work for a horse who is normally quite on their forehand will be very hard work for them and they will get tired muscularly quite quickly, especially in the trot and more so the canter, so to keep it a good experience it is an exercise to be built up over multiple sessions, spending maybe 10-15 minutes maximum per session on it.