I have a 3yo filly which has very low energy levels comparing to other horses. Now she is 7 days at the farm and her energy is still F. Another filly that ran in the same race last time, has A energy since a few days. Her fitness is still A, but she also loses more energy during training, comparing to other horses. She won one race, was once second, once third, and once fourth.
What does it mean? Can she be immature (she was born in April)?
It’s a created filly:
finalfurlong.org/viewhorse.php?horse=21575
All horses regain energy at different rates - immaturity doesn’t factor in to how fast energy is regained. To go from 0% to 100% can take 5-25 days. However, if your filly worked extra hard in her last race, it could mean she’s at negative energy, so it would take longer than normal to get back to full.
Horses at the farm regain energy a bit quicker than not at the farm, but it’s not an instantaneous cure. 
Thanks!