From your description I am assuming you are doing double sided reflow and parts are dropping off the bottom side when the board is run through the reflow oven the second time. A few ideas to try:
1) Are the parts that are dropping off heavier parts, such as big surface mount electrolytic caps? You can glue the heavier parts when they are run the first time through the oven. You can also reverse which side of the board is run first so that the side with the heavier parts is run second, which puts the heavier parts on the top of the board.
2) During the second pass of double sided reflow, the solder on the bottom side usually gets above the liquidous melting point. Above that point it is the surface tension of the melted solder that is holing the parts onto the bottom side. We have developed reflow oven settings such that the bottom heat is set lower than the top heat in our 2 hottest reflow zones where the solder is run up above the melting point. This has the effect of reducing the amount of time the bottom side solder is above the melting point. Our results show that this can help with the problem of parts dropping off the bottom side. We have found however, that this not a common practice so profile your boards to make sure the top side solder joints are reaching proper temperatures for the required length of time.
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