Last autumn, the Piccadilly line lost more than half of its fleet due to
wheel flats. Now, the line is suffering from a train shortage again. For
example, this was tweeted tonight: "Severe delays: Acton town - Heathrow
all terminals & South Harrow due to a temporary shortage of trains".

And, guess what, this turns out to be down to the RMT's not liking LU's
attempts to fix the previous problem:
http://www.standard.co.uk/news/transport/tube-delays-piccadilly-and-victoria-line-hit-by-chaos-amid-warning-of-weeks-of-delays-a3597311.html