Emulsion sauces are easily broken.  If the egg yolks appear to be too thick the heat is too high, cool the mixture down with a few drops of cold water.  If the egg yolks are foaming but to not thicken over the heat , the heat is too low, so increase then heat slightly.  If the sauce curdles at the end of the process the mixture is too hot or cold.  If too cold put a small amount of hot water in a bowl and gradually whisk the sauce into it.  If the mixture is too hot, follow the same process starting out with cold water.  It should be remembered however that bringing back a sauce that is broken will give you a different result.  The sauce will be much heavier than one that has been prepared correctly to begin with.