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OK. You know what's up. Still, the 10dBi really should work fine over such a short distance - with clear line of sight, so I'm imagining that the real issue is that you are trying to punch through trees or something or else, as ChrisJoy says, you don't have a clear fresnel zone.
Or, you've got some significant cable loss going on with your 10dbi, such that you're only getting 5-6 dbi effective.
Instead of a high gain directional, consider a more distance appropriate backfire style antenna mounted up high to get clear line of sight and then connect it with a low loss LMR400 cable.
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That should deliver a real 12+ dbi to your radio.
Another way to approach it would be to put the radio right at the antenna with one of these guys:
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8 and 10 dbi don't sound like much, but you get 100% of it to your radio. And a good radio at that.
Steve