So, I am sitting here and thinking of all the reasons that are possible that we have not yet seen the RFO on Catherine.
On the she has it and is not wearing it side of things:
-The Ivory issue. In this scenario, either the queen understands (which I think she would) and agrees that it is politic for Catherine to appreciate the gesture, but to avoid the brou-ha-ha
-It's a piece previously owned by another royal, returned to the Queen and the Queen has asked for sentimental reasons the Kate not wear the piece. This makes more sense if (see below) Charles has decided he will not have a RFO and so the Queen let that influence production of new orders.
On the she does not have it side of things:
-The ivory issue impelled Catherine and/or/William/Charles/Harry to proactively discuss the issue with the Queen and she agreed - no order for Catherine.
-She has run out of the original RFO and some combination of the original artist was gone/the new artist failed, she decided early in her reign to only have as many orders as she has given out (they are not like sweets, after all). This also works for a RFO produced on porcelain or bone that did not meet family standards. In this case, Catherine understands why she does not have the order.
-She has been down a very rough daughter in law road and in hindsight learned that caution is a good thing.
-She determined long ago that only her son's wives would be eligible - not the next generation - RFOs are not sweets, after all. For all we know she may have hated hanging all those predecessors on her evening gowns.
-Charles has decided he will not have a RFO and so in whatever combination of ivory/production issues she has decided Sophie's was the last order. BTW, I can make a case for the end of the RFO in the UK. The meaning of an order specifically for female family diminishes in a modern monarchy and with modern rules of succession. At the very least, I think the RFO changes in intent and distribution moving forward.
I could come up with more - but my point is that in the grand history of the monarchy, the RFO is relatively modern and used more and less aggressively by different monarchs. And entirely at their discretion. JMO.