ESC only goes fast to very fast
Is the voltage high enough -- Pi will output 3.3v by default, and your ESC may need higher voltage. Thresholds vary by device. You can use simple 555-based amplifier to increase the voltage if you need to.
Are you sure your signal shape is fine? Some devices are more tolerant to bad shape than others.
Do you need to calibrate your ESC? Normally, they come up from the factory with 1.5 ms center, but if someone else has used it before, they may have re-calibrated center to 1.57 ms.