As you've pointed out, voting isn't a numbers game (though it can be in a protest vote but a GE is not seen that way). Also because of voters' mistrust of politicians, telling them not to vote for the Tories can easily backfire and asking people vote to negatively (e.g. stop the Tories) is demotivating. On top of this we need to understand that the Lib Dems are not progressive and neither is a big chunk of Labour. Many Labour MP's have a much stronger sense of self-preservation than any desire to get into government and are much more comfortable in opposition where they can line their nest and make alot of noise but do bugger all politically.