Mary-Kate Olsen was willing to reprise Michelle Tanner in Fuller House, but Ashley was opposed to it from the get-go. The famous twins alternately played the youngest Tanner daughter in Full House, becoming household names as a result. The pair has long retired from acting, which was why both didn't return for the Netflix spin-off — but as it turns out, at least one of them was ready to revisit the role.
Fuller House was able to bring back all of Full House's main cast except for the Olsens. Leading up to the show's release in 2016, there were conflicting reports on whether or not the twins would participate. The producers kept their hopes up, saying that conversations were ongoing but that they felt very positive about the chances of the Olsens agreeing to appear. Even when Michelle was absent from Fuller House season 1, there were still murmurs about how the twins might finally join the cast in season 2. At that point, Fuller House didn't shy away from poking fun at Mary-Kate and Ashley's supposed refusal to join, which left a bad taste in some people's mouths.
Regardless of what was going around the rumor mill, there were supposedly official talks between Fuller House producers and the twins. Mary-Kate was apparently open to the idea and was even volunteering herself to play the part. That's because Ashley was skeptical about reviving her acting career from the get-go; according to executive producer Bob Boyett, she pointed out how long it has been since she last stood in front of a camera to act and that she just doesn't feel comfortable acting. Simply put, Ashley outright didn't want to do it. With that understanding, Mary-Kate almost took the role just so Michelle could be a part of Fuller House, but, despite this, it boiled down to bad timing. She was eager to participate but reasoned that it wasn't going to fit in her schedule. It's worth noting, however, that according to Fuller House star and executive producer Candace Cameron-Bure that they kept asking until season 3. So, at this point, if Mary-Kate was really eager to do the show, they had three seasons to sort out this issue.
As ridiculous-sounding as the show's explanation about the youngest Tanner kid's absence in the series, it's almost the same reason why the Olsen twins didn't appear. In the Full House spin-off, Michelle was based in New York, hence why in the five years and even through the biggest family moments, she never joined her family. There wasn't even a phone call with her, although, at one point, Danny, Jesse, and Becky supposedly spent the holidays with her on the opposite end of the country. Like their break-out character, Mary-Kate and Ashley have mostly stayed on the East Coast and based on the former's explanation, it doesn't seem like a six-hour flight to do a cameo was worth the effort.
Despite not working out, Mary-Kate's stance is backed up by John Stamos' comments on the matter. Like Cameron-Bure, the actor was also an executive producer in the show, and considering his special relationship with the Olsen twins, at least during their time working on Full House together, he privately talked to Mary-Kate about doing Fuller House. As with Boyett's claim, she was open to the idea; Ashley but wasn't keen on it. Still, Mary-Kate asked Stamos to personally reached out to her sister, which he, unfortunately, didn’t.