Hewsu is correct... the most likely cause is water depth. What you should do, is run the mission in "Normal" difficultly mode (so you have an external camera) and then zoom down to the bottom of the icebergs to see if they are touching the ocean floor. In some cases it may also be "clipping" the bottom too, which also causes major problems. Also avoid placing the icebergs too close together, that will cause them to interfere with each other.
There is no easy way to figure this out before hand, although you could place some "test icebergs" to get an idea of the minimum distances, and then go from there to populate the rest of your ice-field.
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