I know what you mean. You have to be really strong and tell them not to remove the coin. If they do remove it, insist they get the manager and talk to the manager. Although they have removed the coin, they can not prove it was on the line but then again you can't prove that it wasn't. There is a common knowledge that the customer is right unless proven and in this case they can't so they have to give it to you. I have done it before.
Another way is yell at them when they try to remove it and tell them to use the camera to proof it is on the line. All Jumpin Gyms have them. If the staff refuse, ask to see the manager. Trust me, the manager are always nearby and there is no hassle to call upon them. They are use to it cos there are more trouble at the rainbow then anywhere else.
If you are friendly with the staff, usually they give it to you if you are on the lines of the red ( cos the prize is small ) but if it is the big prize more difficult cos they need to consult the manager.
One advice though, some jumpin Gyms polish the surface of the rainbow polish so that it is very difficult to win. If when you throw your coins and they slip off very quick, then it is polished and don't waste your coins. It is easier to win when the surface is not polished.
Good luck and yes you can win the big prizes cos I have and so has my aunties and friends.