ViClose takes 15 seconds to return

I've checked the forums and found an equivalent post : http://forums.ni.com/t5/LabWindows-CVI/ViClose-is-taking-15-second-to-close-the-session/m-p/809879

Unfortunately, nobody responded to that post so here I am with the same problem. I have 3 sessions opened at the time viClose is called; one to the resource manager, one "Tx" session to a serial instrument, and one "Rx" session to the same serial instrument. I call ViClose on the resource manager to close all of them at once. The following is output from NI Trace

1960. viClose (0x00B1FA70)
Process ID: 0x00000FE0 Thread ID: 0x000000D0
Start Time: 19:09:15.718 Call Duration 00:00:15.014
Status: 0 (VI_SUCCESS)