You might wanna check if your light have any input connection that control your intensity. sometimes you can accidently connect stuff..

Also check if your lights attributes have been locked (will appear grey in the channel editor) - if that is the case, then select the attributes and right click them and select unlock ...

You could also check if your light have any animation curves attached, if the curve is set to be a "Step" curve, then your light will not change until suddenly and you might wanna make the curve to be a smooth or a linear curve..

That is all I can think of at the moment.

Hope you figure it out, good luck.

Kindly