Lower Body Flexibility vs. Increasing Weight

Hey guys,

I had a question about whether or not for things like squats/deadlifts if I should wait to become more flexible at the current weight or if I should continue to increase the weight when it becomes easy?

I can get approximately parallel with squats but because of the sports I did when I was younger my hamstrings/hip flexors are tight and I can only get just below my knee for deadlifts.

K&D usually say to respect the range of motion but they also say to lift heavier if we are able, so I'm not sure what to do in this situation.

Thanks!