My heated build platform won't get hot enough?

In the lead up to the first Rise of the Machines workshop, I had 15 robot kits I needed to pre-print. However, I was getting a little bit stressed because, for the life of me I couldn’t get my heated build platform above 50°C. It was taking ages to heat up, and when it got close to 50°C the power to the heated build platform (HPB) would cut out, and refuse to switch on again.

This is a companion discussion topic for the original entry at