New waste tank

After fitting a new waste tank we have a problem with waste pipes going into the tank. There seems to be little flow going into the tank and backing up into shower basin.
To get a good flow going through the pipes to the waste tank we need to open the valve on the waste tank. Everything flows perfectly then. We see in the manuel that both fresh and waste tanks have breathers on. Could it be that these haven,t been replaced making it impossible for air to get out and water to get in.
Any ideas would be welcome as we are due to go away on Tuesday!!!!!!!!

Hi
    I would say that you a very much correct,the breather could may be have a non return valve fitted that has stuck
regards
Alan