Yes, I think lack of motivation
Similar to Sloane, but not as bad.
Many other players (Top players) just have more will to win than Garbine
Look at Barty, Halep, Bibi, Svitolina, Pliskova, Kvitova, Serena, Bencic, Sabalenka, Bertens, Kenin, Mertens, Yastremska, Gauff, Martic, Görges and Sakkari.
And then compare them to Muguruza and Sloane and their fighting spirit/motivation
A shame because of their big talent. But Garbine and Sloane simply don´t have the fire consistently.
Garbine and (especially) Sloane give up too quickly and do not fight hard enough.