Have you actually tried distances farther than ~25 feet? The radiation pattern of the antenna may be such that it will work once you get farther away.
Higher gain antennas, even omnis, do not necessarily have exactly symmetrical globe-like patterns. They may focus the energy in rings of waves ... where each part of the wave is either a null or strong signal.
If the antenna is on a roof, try walking out to ~300-500 feet and see what the signal is.
The other possibility may be the cabling. At 2.4 Gig, cabling is critical(!) and should be as short and as high-quality as possible. Common coax (high-quality rg/6) will lose more than half the signal in ~25feet (something like -12 to -14 db per hundred feet). The less you feed the antenna, the less signal you have to "boost" with your 12 DB of gain.
ONE flaky connector will severly degrade the signal.
Using "adapters" to connect the RP-TNC to other more common connectors on common cable will also degrade the signal (in addition to the additional loss of common cabling).
Remember that the 12db of gain is on the signal fed to the base of the antenna ... if you've already lost most of the signal by the time it gets to the base of the antenna, you're not really "gaining" anything.
Good Luck
Scott
