Two thoughts that come to mind:
1) If at first you don't succeed, try a bigger [insert tool/accessory here]. If regular pocket hole screws don't have sufficient bite, redrill the hole for an equal length, but heavier screw. In otherwords, if a #7-8 screw does not cut it, try a #12-14 wood screw. A #14 wood screw has tremendous bite in most materials when installed in a properly sized hole
2) Assuming this is a relatively wide (in cross section) miter joint, have you considered boring one or two holes clear through the joint and gluing in some long dowels. You can then cut the dowels flush to each face with a flush cut saw or sharp chisel. You can purchase ready-made dowels in 12-36" lengths at most BORGs and Hobby shops, just be sure to measure these dowels before drilling holes because they are often slightly oversize and will require a similarly sized hole. If you need/wish to reduce the diameter a bit you can chuck them in your drill and either resize them with sandpaper OR you can drill a hole of the target diameter in a piece of heavy sheet metal (mending plates work well for this) and then manually taper the end of the dowel, they use the drill to 'thread' the dowel trough the hole repeatedly. The end result should be of consistent diameter but it need not be pretty since rough dowel sides will help to distribute glue through the hole.
3) You could create a jig and either use a [STRIKE]crosscut[/STRIKE] (memory lapse) rip saw (or whatever you have) or circular saw to cut your spline slots in an oversized piece. Routers work as well but they are usually more depth limited.