Unable to resign players prior to Free Agency

[user7777]

I did some tidying of my prior question as it was a bit vague, but I have played through a couple of seasons using Historical mode from 86', I'm unable to sign any of my veterens before they leave for Free agency. First year I thought it was because I was over the cap so bye bye Larry Bird, however second year I wanted to keep Dennis Johnson but I was under the cap and could afford to resign him however never got the chance. I was looking at Rosters and then Contracts and there are buttons that highlight for "extend" but after checking into this several times prior to Free Agency period I was never able to extend any contracts, the button remained unusable.

So...is this just how the game runs? Or is there a way or a specific time span when you are able to redo or extend a contract? I am playing with a hard cap
Thank you for reading, my first post in probably five years when I owned DDS2