My imac not recognize my iphone 4s

My imac (OS 10.6.8) does not recognize my iphone 4s running ver. 5.1.1 when i connect via usb. Niether itunes or phoneview recognize the phone. The phone is connected as it is charging. Have tried all usb ports. other devices (printer and external HD) work on same ports. The phone is recognized on my old computer (powerpc G5) in both programs. I have authorized itunes to recognize the imac. All suggestions gratefully received.