SMT diode

We have been experiencing a reflow issue with a SMT diode 4007. This is a SMA or DO 214-A package. It is a lead free part used with 63/37 paste no clean flux. The problem is it is not consistant. 100 diodes can be reflowed rock solid on the board and then one or two can be flicked off with with your finger with little effort. The solder joint on these looks a little grainy, like a cold solder joint. Peak temp is about 225 C on this diode and about 70 seconds in liquidus. Trying to narrow down the root cause is pretty difficult. Going by Lot # is not helping much. I do have a picture of the solder connection after the part has been "broken off". If anyone can tell me if they are having this same issue please let me know. E mail me if you can provide some possible avenues on where to look for a root cause.

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