I have recently installed a new toilet upstairs, and the water level in the basin is constantly too low. When the toilet is flushed, the water disappears very quickly, and then goes back slowly to its original level. The other toilet I've installed in the room next door works fine. Both soil pipes are running at a 90 degree angle under the floor boards before going out through the back wall and down into the drain. There is no stench pipe for these toilets, but would that cause the water level to be so low?

If you could help that would be great.