Thank you all for your responses about this issue. Main points were:
 - temperature (too high or too low)
 - flux activity
 - contact time
 - possible shadowing
 - clealiness of the pads
 - diffused nickel
 - turbulence

I still haven't checked all that though.

Does anybody know about a good course/tutorial for engineers about wave
soldering ?  I need it.
I checked with IPC, EMPF and SMTA and I didn't see anything in a near
future.
Thank you.

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