Thanks for the responses guys. Lesson learnt.

Bought a brand new bag yesterday and it went on like a dream.

Maybe this is why have found it difficult sometimes in the past, I've
never noticed the use-by date before. I usually look at the job
upfront and try and buy everything I anticipate needing for the job.
Plastering is one of those jobs I don't really like doing so I often
put it for as long as I can. In future I'll only buy the plaster when
I'm ready to use it.