OK, I have been having that happen lately too.
2. if you don't already have one, try a hands free bra so you can do compressions and massage
3. try again in half an hour.
4. do the stimulation phase thing again and try again
5. try some mother's milk tea for the marshmallow, which is good for letdown
6. check with medela
7. replace the valves and membranes anyway.
8. try a different size horn
It's terribly frustrating. That happened to me all weekend. I was over it