I think you are just not 大方 enough.
Go ahead with the outing, but don't invite that senior if you
want. Maybe it will spoil the friendship if he found out, depends
on how close you all are.
Else you can invite all, then during the outing don't make
yuorself invinsible that is, talk and flirt also la. There is
nothing wrong with the senior lor. It is you. You should flirt
more.
Then your next
question will be, how to flirt?