Pairing FPGA with a QSFP range extender

I am trying to connect the FPGA with a QSFP Range Extender from 10Gtek (http://www.10gtek.com/qsfp-extender) (CVT-OEO-Q/4S1C)... basically the QSFP port from the FPGA goes into the range extender and the SFP ports from the extender can be collected at the destination.

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In this setup I am seeing very inconsistent results, that is sometimes the links come up but sometimes they dont. 

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If I try to connect the FPGA to the destination using a DAC QSFP splitter cable, then things work just fine.

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Also I have tested the Range Extender with an Arista switch. That is the SFP from Arista goes into the QSFP of extender box and it is output goes back into Arista. This setup works fine as well.

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Only the FPGA + QSFP extender setup is flacky. Please help with this issue.