BQ51003: I don't get a Qi lock on all PCBs in the same batch.

Part Number:BQ51003

I have used the suggested BQ51003 circuit from datasheet   (and development board)  but only get a lock on around 5 out of 10 PCBs in a prototype batch.

3  different  off-the-shelf  Qi   TX units do the same.

Attached :- 

-  RX  circuit schematic  jpg

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-  datasheet for the RX  coil being used

-  signal on BQ510003  output  where you can see it starting to talk and get up to speed but then turns itself off and starts again

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-  feeds  MCP73833  battery charge controller with limit set to 200mA  

-  still no lock if the battery is fully charged and the circuit is turned off so that it's virtually no load

Any ideas for what I can try to fix this?

Thanks.