Maybe it relies on the more volatile propylene for ignition, and this
has evaporated / leaked / diffused through the seal leaving you with an
apparently full can containing only butylene?
I have a vague recollection that some light unsaturated compound
polymerises fast enough to "go off" in storage at a significant rate.
Need a proper organic chemist to advise.
Looking again at the MSDS, the contents are described as fuel and
propellant, tempting to say butylene is the fuel and propylene the
propellant (although both will burn, and the butylene has some vapour
pressure).
So loss of propylene will also cause significant loss of pressure,
perhaps the problem is that you are not getting an explosive mixture
from the old cylinders.
You might try warming up the cylinders gently, especially in cold
weather. I'm sure they are labelled with something like "Don't expose to
more than 30 C" but they are not going to explode on you at (say) 40 to
50 C. Stick them in a bucket of warm water to give them a chance to get
up to temperature, outside if you are a bit paranoid.
I don't think smell is a clue, I'd expect both compounds to smell similar.