If you have a tank type of toilet as opposed to a flushometer then lift the tank cover and put the refill tube back into the over flow to replenish the toilets trap seal.

Also check the fill to this level line.

If all else fails try snaking the line as you may have a partical stoppage

Also if the toilet is a different model/brand then the others and has been replace a number of newer toilets have a small "water spot".

That is the area that is covered with the water.