Hi @barmstrong
Low water level in a toilet bowl is indicative of a partially blocked sewer pipe or air vent.
If the other toilet is filling instead of draining this also indicates a blockage in the pipes somewhere.
You could try clearing it using a plunger.
If still no good then you may have to try using some other options¹ or calling a plumber as they should have the necessary equipment to clear drains.
¹Option shown only to show example of what you can use - not an endorsement of the service.
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