Working with minecarts unloaders, I noticed that minecarts don't always move when power rail is activated underneath them.
If the cart is touching a block or some part of it is over a normal rail, it will move when the power rail is activated. But if it’s completely over the power rail, it won't move when rail is activated.
Minecarts have always worked this way in all editions. The game does not know which way you might want the cart to move when you power the rail. The cart needs to be given initial momentum by an incline, nudge, or being near a block.