Take a 5 gallon bucket and put 3-4 gallons in it. Pour it in the bowl as fast as you can. If the toilet flushes normally then the water ways are restricted with mineral build up and the unit may need to be replaced. You can try acid and try using a coat hanger and mirror and open the holes under the bowl rim but these are normally temporary at best.
If you pour the water in and it does not flush normally you have a stoppage in the bowl or drain line.