it is possible to put the cas on incorrectly and it be out one or two teeth. what timing light are you using? where are you getting the signal from? have you tried a different timing light?
if your confident your timing light is reading correctly you could try, unbolting the cas, pull it off, dont turn cas the shaft yet, have a look to see if the half moon shaped locating pin on the end of the cam shaft, is missing or not, if it is there the cas will only go on one way.
if its not the cas may have been put back on incorrectly so its out. then with the cas held so the shaft is facing you, turn the cas shaft anti-clockwise a little bit so when it goes back on it is one tooth advanced.
then put the timing light back on it again. you may have to move it more teeth to get it right.