1) Too much water (inhibits imine formation)
2) Too little methylamine (there isn't enough for the imine to form quantitatively)
3) Too short time for imine to form before reducing agent is introduced (I have always wondered why people doesn't let the imine form first, and then reduce the pre-formed imine with Al/Hg)
4) High temperature (imine hydrolysis, more aggressive reduction)
