Long delay in license checkout

I have 3 FlexLM servers for our 2 floating licenses.

While license checkouts work, there is a long delay before the checkout, which makes compilation very slow:

time icc -V
Checkout succeeded: Comp-CL/<...>
        License file: 28518@<internal IP>
        License Server: 28518@<internal IP>
Intel(R) C Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 16.0.3.210 Build 20160415
Copyright (C) 1985-2016 Intel Corporation.  All rights reserved.

real    0m25.153s
user    0m0.004s
sys     0m0.007s

I saw that people have had similar problems e.g. https://software.intel.com/en-us/forums/intel-c-compiler/topic/606143?pa... but didn't see a resolution.

There was some indication this might be to do with IPv6 but I've disabled that on the master license server and that hasn't helped.

This is with CentOS 7.2 on both servers and client.

Thanks,

Adam