I like to do diagnosis of 1911 problems as a process of elimination.
Since it's a stovepipe, ammo has already successfully fed and fired, so you can generally rule out mag problems, underpowered bulk ammo and guide rod springs (unless springs are recommended to be replaced every X,XXX rounds).
Since it has always ejected properly in the past, you can generally rule out ejector length.
Which, by elimination, says the extractor's not doing its job.
Could be either dirty, broken or weak. If you feel competent, remove it (inspect for broken) and clean out the tube. If this doesn't work, then you know its weak. If it is adjustable, do so and test fire. If not, then replace it.
Good luck.