Fuel not Returning!!!

There seems to be no returning fuel to the tank. All hoses seems correct and rightly placed back. The car just could start and so the mechanics checked the incoming fuel it was alright. The returned hose at the tank was detached to check, so they started the car to see but there was no returning fuel coming out from the return line. They replaced the return valve but still there no return fuel to the tank. The mechanics suspect that this might be the cause of the unsuccessful starting attempts and the immobility of the vehicle. Two questions arose 1. could this be the main cause or just partly it? 2. what could be cutting out the fuel's circulation???

Any advice on this is highly appreciated.