Hit the why bother wall

I work in and office been there 5 years i like to think I'm good at what I do as I tend to finish my work and get handed other people's work where my frustration lies is I get asked to:

1.Source new products for purchasing department - head of purchasing just go's that's nice but I'm going to get this instead and I sit there and wonder why ask for help then

2. Systems asks to help streamline processes in other departments as I have better systems knowledge then most - do there own thing again anyway and don't take on feedback or recommendations again why ask

3. Shop floor manager asks can you get me some quotes for this new tool - do up a list with new and existing suppliers decides nah let's go with this other one not on the list we haven't checked out

For a long time its no or let's do this instead or just plain don't listen and dont provide any constructive feedback has anyone else experienced this