vim seems not loading .vimrc on OSX sometimes

In .vimrc there is a line set expandtab, but sometimes this doesn't work. The problem is When I press tab key a new tab is inserted and it is not expanded to space. I can see the 'tab' is there with set list.

:scriptnames listing shows .vimrc has been loaded, but the problem is still there sometimes. and I have to run set expandtab by hand each time.
1: ~/.vimrc

And there are other similar issues it seems the .vimrc is actually not loaded. Any idea what is problem?