While Jon and Sansa are campaigning around the North for forces, they're taking a tour of some other noble houses and families that viewers have yet to see up close. One such family was House Glover, supporters of the Starks for generations. Despite those many years of unquestioning support, they were unwilling to join Jon and Sansa. Why won't House Glover help the Starks?
Like many people in the North, House Glover took a serious hit during the War of the Five Kings and they're still recovering. They were supporters of the Starks and Robb in particular, but that support wasn't returned when they dealt with their own conflicts. Their castle was swarmed by the Ironborn, the Greyjoys, and Lord Glover's family was tortured and killed. They didn't have House Stark there to help them when they lost everything, so why should they come to the Starks' aid now?
It doesn't help that Jon and Sansa are lacking any support besides the wildlings and the tiny House Mormont; the people of Westeros are seriously prejudiced, and combining that with the uncertainty of the battle itself means that the entire situation is a really risky proposition. The Boltons were the ones to help the Glovers take their castle back from the Ironborn, and rising against the Boltons could result in deadly consequences.
It might have seemed like a sure thing when Jon and Sansa first went riding out to gain supporters. Sansa was certain that the Stark name meant something to the men in the North and that they would always rally behind it. However, from where Lord Glover is looking at it, House Stark no longer exists. Everyone is either dead or missing besides Jon and Sansa, and they don't necessarily hold a lot of cache with the status-conscious people of Westeros.
The Boltons have also proven themselves to be immensely powerful, at least in part because of how unpredictable and cruel they are. They have no problem cutting down anyone who opposes them, and they've built a considerable army. They've also helped out the other big Northern houses, as evidenced not only by the Glovers, but by the fact that the Karstarks and the Umbers have already switched from Team Stark to Team Bolton.
As disappointing as it is, the Glovers' refusal to help is also understandable. It adds context and layers to the political machinations governing Westeros, even if it's also a total downer. Jon and Sansa have a long road ahead of them as they journey to regain their home and it's becoming increasingly clear that they are the underdogs in this situation.