You only have partial permissions to access her inbox. Instead of delegation, have your IT department give you Read and Change permissions to her mailbox and map the mailbox to your Outlook. Which will make her mailbox show up as a PST file would in your mail folders view.
Your boss should be the one who grants you the access you need, not the IT department. There may be a reason she did not give you access to those specific folders. Get her input on this and what she means for you to have access to in her mailbox. If she wants you to have access to those folders she can grant you that access. If she does not then you know why you cannot open them.