For upgrades, as you upgrade your reactor core, you need to upgrade cooling towers to help cool them down more, (you can also double up on cooling towers if you want, but having 3 or more is less than super beneficial).
Also note, Power isn’t generated from having a high core temp, it’s generated from having a high differential between reactor temp and your cooling tower.
As for contracts, they are a tad finicky, the offers should scale as your power plant grows, so you need to re-click on the contract to find new deals that work better with a larger plant. (I’ll adjust the UI in the next update to make this more clear.)