Well having serviced many ladies watches the smallest being the size of a marble maybe I qualify to have a go at the diagnosis. Back to basics remove the power from the watch and then remove the balance and the fork, Now test the train for freedom by applying half a turn on the main spring, if the train wheels turn freely wit a little back spin when stopping the train is deemed free. Now fit the fork and put one to two turns on the mainspring and check the fork snaps back and forth when moved if that's ok then re fit the balance after check, Inspect the pivots are intact, Check the balance spring is flat and the coils are free and not touching and not sticking together when oscillating, Thats for starters.