I'm no master of UK politics, but it seems like the Lib Dems have had quite a run in the local by-elections, without any noticeable uptick in nationwide polling. Any theories as to why this could be? Is this a common occurrence?
Yes very common. The Lib Dems usually do far better in local elections than they do in national elections and especially in local by-elections. They were doing very well in local by-elections prior to the 2017 GE but then they lost seats in the local elections and their national vote share fell even further at the GE. The Lib Dems have always been a more 'localist' party than either the Tories or Labour (who are very much 'national' parties) and thus most of their wins in these local by-elections are fought over local issues such as potholes, rubbish collection, trees etc. or the general competence (or lack thereof) of the Tory/Labour run council. These wins have nothing to do with national politics and unfortunately for them people don't choose the government based on 'who'd be most likely to fix the pothole outside my house'.
I think long-term improvements in local presence do help the Lib Dems, and it's clear that the Lib Dems do better in parliamentary elections in seats where they have a significant local elections presence than they do in otherwise similar seats where they have much less/zero presence in local elections. The one thing the local by-elections do indicate is that the Lib Dems are not actually dead as a party and will continue to muddle on rather than vanishing into obscurity. Of course, if the standard of success is performing as well as they did in 1997-2010, that's probably not coming back in the next couple of election cycles at least (although the Lib Dems and their Liberal predecessors went through multiple revival periods basically centered on single leaders, and it could be that they are just one successful leader away from another significant revival).
On a related note, two of the three by-elections this week look ripe for Lib Dem gains. The Tories should hold on to their seat in Highland (probably with the SNP second, though I wouldn't be totally shocked if the SNP managed to win the seat), while I'm going to predict Lib Dem gains from the Tories in both Thatcham West and Lymm.
