You understand that the virtual channels come in on the same frequency, from the same real channel? It's unlikely that you would be able to receive 5.1 and not 5.3, or vice-versa. So you must be right on the cusp of reception... maybe there is something about your TV's tuner that allows the SD version and not the HD version at low signal levels.
Yes, get your compass and aim right at 47 degrees magnetic. And remove the splitter. The passive splitter does not degrade the signal very much, but does divide it in half, so that cable losses and tuner sensitivity become more of an issue.
Adding a second amplifier, like the Jolt, is a weird and sub-optimal solution IMO... though I understand that it's Mohu that designed it this way. Normally you'd put the antenna amplifier as close to the antenna as possible, using a mast-mounted (internal to the antenna in your case) amplifier. That is where the antenna signal is strongest, and degraded the least by the cable losses. This means that the added amplifier noise is the smallest possible fraction of the total signal, and gives you a lot of signal voltage before it's cut down by the cables and any splitters.
Another possibility - your antenna may be in a local null for Fox. Look at this site
http://www.hdtvprimer.com/antennas/siting.html and it will give you an idea of how these local nulls can arise due to ground and building reflections: "multipath." Changing the location of your antenna by a few feet may move you out of that null. But aim it first.
Good luck!