Right now, some bad polling in Wisconsin for Trump seems to be revitalizing the Stop Trump movement, such as it is. You can see for yourself here. It is worth noting that Wisconsin was never a Trump stronghold. Governor Scott Walker unsurprisingly led there until he dropped out. There were a couple of polls with Carson ahead during his little boomlet, but no long-term, stable lead. The interesting thing is that Cruz has tended to beat Trump in the rural, sparsely populated states, like Iowa and Kansas, and lost in the urban states. Wisconsin has some big urban clumps, which would have augured well for Trump. Will he lose Wisconsin? The odds now are yes.
Remember, though, defeating Trump requires not only holding him under 50% in the delegate count, but convincing the delegates to give the nomination to someone with fewer votes and fewer delegates than Trump since it is essentially impossible for anyone to pass him in the delegate count. New York, California... Cruz will have a very difficult time beating Trump in the largest remaining states. Trump will be the delegate leader at the convention, and he is only denied the nomination if the party decides to blow itself to smithereens to avoid a Trump nomination. That's a tall order...
... unless his numbers tank at the national level. That could happen. If the Republican electorate in states that have already voted (as shown in the national polls) turn heavily against Trump, that could help the Stop Trump movement pull off the coup with minimum bloodshed.
Point being, if you want to know whether or not the conventioneers can stop Trump, watch the national polls. A shift there is the only thing that could give the party cover if they try to steal the nomination at the convention.