If they were in the window box all last summer and over the winter they may be pot bound and the soil depleted of any nutrients--in fact maybe what you have is roots and no soil. Cutting back won't help.

I bring some Geraniums inside every fall in the same pots they grew in all summer, but when they go out again the next spring, they get some new soil and a bigger pot.

Dig them out of the box and look at the roots. Mix what soil there is with new soil and replant. You may not be able to put as many plants back in. Your plants are bigger than they were last summer and they need root room. The bonus is you can plant them elsewhere.