What you're referring to isn't actually the seat belt pretensioners (those go bad when the airbags deploy), but rather the Emergency Locking Retractor (ELR).
Pulling it out slowly is the typical response. Sounds like you know that already. Whenever I have someone else drive my Subaru, I sometimes have the same issue. I re-attempt with a slower action, until I get it right. For some reason, that passenger-side seat belt ELR is more sensitive, but I haven't bothered to get it checked.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano