It soundl like you are haveing a lign of sight problem, i would suggest these long range 30 km units . I have used them before and with 2 of them 2-3 km non line of sight it will connect. Set one on Ap and thew other in wireless client mode, from
www.zigwire.net
Product Name:AMU-2405G-SMA
Product Class:Long Range Wireless LAN system
Features: AMU-2405G-SMA 802.11g/b 500mW , AP/Client/Repeater/Bridge; LED Wan/Lan/Power; Lighting arrestors protection; Stainless Steel adjustable mounting bracket;No internal antenna panel; Plastic outdoor weatherproof case; -30c~65c ,120/240~48vdc power ,up to 30km with Yagi, 16-dbi gain[not included]
Details...
Long Range Wireless LAN system---Zigwire