toilet problem

when i flush the toilet, it won't flush all the time correctly, and sometimes the bowl will not fill up with water like it should. i checked the tank, and the water seems totally fine there (the water level). what could be the problem why it isn't flushing right? (i sometimes have to hold the handle down long for it to flush right).
why isn't the bowl filling up with water all the time? is there something trapped down the line?
thanks for any help
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