I also had 2 vac schemes last cycle and failed to convert.
I'm convinced that law firms give you one chance, so make your first application to them the best.
(I'll caveat that with, unless something drastically different has happened in that year and your next application will be much much much stronger).
Each time, I've had my most luck with firm's I've never applied to before. I think they try and give people a chance first time round.