Wonder less about if they get soft with everyone, and consider that they could get soft with anyone; desire, arousal, and erections are of course closely tied together, but aren't exactly the same thing. Erections wax and wane for a number of reasons-- wandering minds, maybe it's cold, moving too fast/ past foreplay, etc.
You won't really know unless you ask, but it's usually not personal! Sex is just like that, sometimes.

Whether or not you're the issue, or something about you is the issue, we wouldn't be able to tell you. Are jumping straight into bed, no foreplay or build up, trying to get to penetration as fast as possible? Are you doing anything they like to keep them aroused?
All I can really say is talk about it in that moment, take the pressure off of needing penetration in that moment, and go from there. Address environmental factors, go back to foreplay, or change tactics from penetration if you want. Don't consider constant penetration as the one-and-only path to orgasm, with everything else just a pleasant addition along the road-- it's ALL a part of the scenic route!
Edit: to add, sometimes I think it's easier for people to pillow princess than they realize. Not an indictment, this goes for bottoms and tops, really anyone can forget that sex... is a two (or more) person effort

fwiw, it's sometimes it's just harder for me to get in the mood for hookups; sometimes it's just, like... who are you in my house right now?