Player retention will be affected by league performance and budget constraints. Take a look at the season Atletico Ottawa had compared to Valour. If you were de Brienne and Yesli, where would you want to play? On the other hand, if a player has the sort of season like Antonoglu is having and a "bigger" team comes sniffing around, it is a no brainer unless you are constantly pushing for top spot.
On the budget side of things, Valour finish bottom and offer player 'X' a certain amount to renew because it's all they are allowed, if someone like HFX offer the same or slightly more, where would you go?
Which brings me to recruitment. What sort of scouting/player recruitment setup do Valour actually have? Are we not finding good enough players due to not having the scouting reach? Or because we can't afford them?