hello, the main reason we get this a lot on why the tiny umbrellas wont stay up are 1 of two things.
Firstly can you make sure you do not have your thumb on the release button, place your finger and thumb either side and push it up to the top until it clicks in place.
Secondly customers being to gentle and frightened to push all the way up until it clicks in place. Give it a shack and push it all the way as far as it will go until it clicks in place.
These two things sound simple but in 99.9% of cases it is one of these two reasons.
Let me know how you get on.