Was going to try and save you money and tell you limescale isn't the problem .. But TB beat me to it.
What I sometimes find on cheap taps, is a steel screw is fitted, this will rust inside the brass spindle and wont budge. With a good tight screwdriver forced into the screw head .. turn the tap head back and forth while forcing the screwdriver into the head and snap the screw off. Then it is a simple case of gripping the spout with a pair of grips and undoing the jumper body with a ring spanner.
.. This is assuming you actually have a complete reviver kit and not just the acrylic heads.
If you do have just the heads then you may find they dont even fit your existing taps or maybe even a sloppy fit .. but you wont find out until you have butchered your 'stuck on head'.
Whenever I fit reviver head I always get a complete kit with new jumpers, this eliminates any poor fitting acrylic caps.
In the good old days we had nothing but British made taps .. I have repaired taps a hundred years old with parts out of my current tool kit, just by refacing the tap and fitting a new washer and re-packing the gland. But now we have much poorer quality taps coming from every corner of the globe, sadly it is often quicker to replace the whole tap and fit what has become standard these days .. the contract tap .. a cheap disposable unit which is available in every DIY shed.
Have look at what a British made tap costs and then you will see why we buy the alternative.