Once removed, if the manhole is clear, the blockage is between the manhole and the house. The only effective tools for this job are a set of drain rods because they are flexible, unlike a stick.
Sometimes a small blockage could be cleared by plunging with a mop or use of a stick, but the outlets and inlets are hidden by effluent and flexible rods are usually required.
Screw two rods together and slide them into the chamber in the direction of the suspected blockage. Screw on another rod and continue pushing, whilst manoeuvring the rods vigorously back and forth. Always turn the rods in a clockwise direction as you push them, this will stop the rod loosening or a joint undoing whilst underground. Continue until you feel the blockage clear.
The chamber should then be flushed thoroughly to remove all traces of debris. Sometimes the blockage is caused by a build up of fat in the chamber. Flush hot soapy water through the chamber to help remove any remaining debris.
If the blockage cannot be located, it must be further down the run. Lift the next manhole cover and repeat the process.
If the lowest manhole is blocked, then the blockage must be between that manhole and the sewer. If it is an interceptor trap, the blockages are nearly always in the trap.
The trap is usually full of effluent, some stoppers are on a chain otherwise feel for the handle with a stick and try to hook it out of its socket. Try not to let the stopper fall and block the trap. Use the rods through to the sewer and then flush out the channel with running water to clear debris.
Replace the stopper and before replacing the cover smear a little grease around the frame to stop any rust in the future.