What I like to do is to take a mop bucket full of water, pour it into the bowl. This will tell me if there is something wrong with the bowl or the tank. If the toilet flushes normally, then there is a problem with the tank to bowl. If the toilet is old, calcium could be a problem. Another problem is toilet bowl cleaners, sometimes all or part of a toilet bowl cleaner container or brick can flush down into the rim of the toilet. Since the rim is then partially blocked, not enough water at the right speed can get to the bowl, so it doen't flush. If its a new toilet, the settings must be exact, the fill valve has to provide enough water into the bowl through the overflow tube to crest the water so it will be at its maximum height when the contents of the tank are emptied. In addition the water level in the tank has to be at the setting shown inside the tank so it will have enough volume to flush. Typically this is one inch below the top of the overflow tube.
If you find when you pour a bucket into the bowl it doesn't flush, there is something in the bowl blocking the trapway. This can also be calcium, or caked on deposits from stools. If the toilet never had a ceramic trapway, debris and effluent can harden on the inside. You can clean some of this out with santeen, but if the toilet is too old, a new one is a cheep solution. But never buy a cheep toilet, Kohler or American Standard are the best.