An upstairs toilet can clog just as often as a downstairs toilet, even though it should have a gravitational advantage with the drainage system. Although the downward flow should give the toilet more flushing force, problems like broken mechanical parts or a drain clog can make the toilet water go down slow or not at all.
The upstairs toilet may go down slow because of a flushing operation malfunction within the tank itself. The flapper valve covers the hole where the tank water goes down, forcing your toilet to flush. If the flapper valve doesn't open all the way, not much water goes through to generate the flushing force needed. The flapper valve is opened by a chain connected to a flushing arm. If the chain is too long or broken, it may not be lifting the valve up properly.
The toilet could be partially clogged with unseen objects, preventing toilet water from going down the drain pipe. The clog could be caused by a mass of toilet paper or other soft objects that, when bound, becomes a more solid object that interferes with water flow. The clogging object can get stuck inside the toilet trap, which is designed to catch these objects before they damage a drain pipe.
Other objects, like small children's toys, can get caught in the trap and partially block the water. They can be removed by hand or using a toilet auger. The auger has a flexible line with a hook at the end that you can stick into the toilet bowl hole. You "snake" the auger by manipulating the turning handle back and forth until you loosen the clogging object.
Clogged Drain Pipe
Although an upstairs toilet should have a good gravity feed for flushing, a clogged drain pipe could slow it down. The drain pipe from the toilet can get clogged if grease, oil and other non-soluble liquids are consistently flushed down the toilet, forming a residue inside the pipes. Over time, the residue builds up and begins trapping other objects like gum, leaving minimal room for water flow. In such cases, a chemical application or hot water may be used to loosen up or destroy the residue.
Backed Up Drain System
If your home or neighbourhood's drainage and sewage system is malfunctioning, it can cause a back up in more than just your upstairs toilet. A backed up system can slow down or stop any household drain pipes. You can verify this by looking at other water-based drain systems in your house like your washing machine or another toilet. If this is the likely cause, you may need to call a professional to determine whether or not it is your home or the neighbourhood sewage system.