I used to have Good Harmonic days and Bad harmonic days. The more peeved off I got the harder I pressed my thumb into the string and the longer I did it for. It sucked. Remember to use light touches. Make it easier on yourself to make them squeel too: give it full overdrive and tons of treble to start with. Also remember that for open strings you can get an idea of where the right places to touch the string are by measuring with a ruler the distance between the nut and the 3rd,4th,5th fret and comparing it to the distance from the bridge where the same harmonics can be produced. This will give you an idea so you can learn to use your pick ups as land marks, the Fifth fret harmonic is around the neck pick up, the 4th the middle etc. Obviously this changes when you fret notes but gradually you pick up a feel for it, harmonics hardly move from these positions if you fret towards the headstock, the pitch doubles but the positions are right if you at the 12th fret etc.
Good luck!