1/ You may have short Achilles tendons. You should be able to bend you knees just over your toes with your feet flat on the ground (when not wearing any boots). If you can’t then you have short Achilles tendons/calfs. This is mostly seen with ladies who wear high heeled shoes all the time.
2/ Your boots could just be too stiff!
1/ The first reason can be relieved with a Heel Raise. This really can make all the difference. Try it now: Stand flat on the floor and bend your knees as far as you can without lifting your heels off the floor. Now do it again with a small book under your heels and see how much further you can bend without your heels coming off the floor!
Ways to help stretch out you achilles/calf muscles: Stand on the edge of a step and let your heels drop down thus stretching out your calf muscles and adjoining body work.
When you are cycling, stand up on the peddles with you bottom off the saddle and a good stretch can be achieved by dropping your heels down.
2/ You probably should buy softer boots. BUT, you could try a Heel Raisefirst. This will make you lean more forward and should enable you to flex forward more especially if you have a touch or reason 1 also.