The difference between Roberto and Bartra is Roberto actually took the one chance to impress Lucho and he did it.
Lucho convinced Bartra to stay in the Summer, but it all went down hill for him as soon as Pre season started. He was a mess during the tour in the States and when he played a bit during the Super cup he was a Disaster.
I think after Verm's Performances and Mathieu getting his shit together again at CB early on in the season it became clear Lucho started to lose Faith in Bartra. Bartra would have one decent game one day and completely shit the bed the next.
Roberto on the other hand the chances he got he impressed everyone and rarely put a bad performance this season so far no matter where he played. Bartra sadly just doesn't have what it takes to make it here and will surely be sold in the summer.