I resigned from the PTA after too many similar experiences. The last straw was when I volunteered to work the book fair. I arrived at my scheduled time, and stood around doing nothing, while two other members sat at the check-out table and talked, ignoring me. So I talked to some of the kids, helped them choose books, etc.
Then I heard the other two members talking about how they had been there all morning and were starved and wanted to go to lunch. So I said I'd be happy to handle things while they went to get something to eat.
After much hemming and hawing, it came out that they did not want to leave me in charge of the cashbox. I cannot fathom what possible reason those women could have had for thinking I would run off with the book fair money (consisting mainly of checks written to the PTA).
But they did say I could go and fetch them lunch! I lost my temper and said "Are you sure you can trust me not to steal your lunch money, too?" and walked out. That was my last interaction with any of the members.
Childish, I know, but I felt really insulted.