Problems with 2000 Volvo V40 WHEELS

2000 volvo v40 front wheels become loose. (1) car was taken to dealership for recall on braking system. (2) departing dealership, rumble from front was noted. two miles from dealership, extreme vibration caused me to stop. the passenger front wheel had nearly come off. (3) car was returned to dealership. i was informed the lugs: nuts must have become loose. i pointed out that technician had taken the wheel off and must have forgotten to tighten it. service manager said this was not possible. a week after my complaint the dealer agree to inspect the car for problems. service manager said no problems were found. after leaving the dealership the wheel became loose again after 75 miles of travel. i retighened the front passenger wheel. also, loose were the front driver wheel and the rear passenger wheel. is there a problem with the lugs: nuts on the v40 series? there are only 4 per wheel. i regularly must check all lugs: nuts. is the dealership using incorrect methods/tools to ensure proper torque on wheels? they deny any wrong doing. *ak