It is helpful to note that our body-proportions can also have a bearing on how accessible we personally find many Pilates exercises.
I have known master-level Pilates instructors who continue to struggle to keep their legs on the ground with the roll-up, but can do teasers without a second thought; this can be simply due to weight distribution.
With heaver legs, especially muscular calves, the practitioner is almost getting the effect of wearing ankle-weights (as they naturally have weight placed further from the point of flexion).
The upside is, practitioners with lighter legs will always have an easier time executing teasers than practitioners with muscular/long/heavy legs!
Keep working at those exercises you find a challenge, but know that somewhere out there is a body who is struggling with the very exercises you no longer have to think about!