A common issue I used to see from clients all the time is that they would report that the buy and sell buttons are greyed out. This can occur for two reasons.
1) As you mentioned, the investor password is being used to sign into the account
2) This one happens more often than the reason above and that is that the client hasn't entered any volume into the order window. Here is an example of how that looks:
They just need to enter some form of volume figure in there and it will light up. Important to note as well that if a client enters a volume figure that isn't able to be sent, for example, 0.01 on AUS200, then the buttons will remain greyed out.