hun i'm not wise or knowledgeable but i think it has a LOT to do with their age.
DD1 is starting to push the boundaries more, telling me NO MUM or snatching things and getting very frustrated at things.

For me i think it is the fact that she seems to 'grown up' for her age that i guess i expect a little more from her than i should, YES she understands when i tell her things but then i think she really is still a little girl who is MEANT to test the waters and find the boundaries.

I don't have any advice hun, i think your doing a wonderful job and it all comes with time.