Oil Cooler Flow Blocked?

[75coupe]

I got a secondhand HKS typeS oil cooler off a GTR, came with all the bits including the sandwich plate with internal thermostat.

As soon as I installed it water temp would rarely go over 75 degrees and temp was fairly much stable at 70, so cool I thought its doing a good job at cooling the oil before install it water temp would usually sit at 77-85, (I dont have a oil temp gauge).

But about 5 days later the water temp was consistently back to its normal temperatures and at a track day the water temp was up to 94... which is pretty much the same as before oil cooler install.

I'm thinking either there is something blocking the oil flow through the core or the thermostat isn't opening properly.

Any ideas on how I can troubleshoot this to define where the problem lies?