Stop using xcompmgr. It's a proof of concept application and was never intended for day to day use. Have you checked the Arch Wiki
to see if there is a better choice for compositing a window manager like Openbox? Something like Cairo Compmgr might be a better choice.
Of course, it might be simply that you forgot to add the ampersand (&) to the end of the xcompmgr line in autostart.sh. Doesn't change my advice to stop using it though.