There are a number of possibilities, so I'll run through what I can think of after having gone through 40 or 50 different TIG coolers in the past couple of years.
First off, any water-cooled torch can have the lining break down and get plugged up, but it's far more common when they sit unused for long periods of time. The normal route of coolant flow is blue out to the torch, and red back to the cooler. The power wire is in the red line, so there isn't really a lot of extra room for crud buildup.
You should be able to disconnect the red line that goes from the Dinse connector to the cooler input and see what kind of return flow you're getting. If the pump is turning, the fan should be running since the motor powers both. Since your cooler doesn't have a flow indicator you might consider adding one...they aren't terribly expensive and can be retrofit from other coolers. More often than not they're the hamster wheel type.
Since you're getting a new torch the best bet is to drain and flush the cooler. I like to drain them, then fill them with tap water, drain and repeat a couple of times....get it upside down, shake it around, etc...try to loosen up anything hiding in the tank. If it's really bad I completely disassemble the cooler to remove the tank and go that route to get it clean.
Your cooler has an in-line filter that should be replaced...I think that model has it pretty close to the filler neck. What isn't obvious on the user manual or parts manual is that the pump also has a screen that needs to be cleaned or replaced. When you look at the pump it will have a large, rounded nut...the screen is behind that. Miller charges a ridiculous amount for them, but I get them on eBay or Amazon quite a bit cheaper. If you buy a couple to spread out the shipping you can get them down pretty cheap...just replace them every couple of times you replace the coolant.
Once the tank is clean, the filter and screen are replaced I like to fill the tank with tap water and hook up an old hose to the blue output connection and run the system so you're pumping clean water through the entire system...all the fittings, the pump, the radiator, until it's obviously clear water flowing. That also lets you check the output pressure/volume. With the output hose held at a bit of an upward angle the water should shoot out at least 5-6 feet...many will send it twice that.
Once everything is flushed and seems to be working I'll drain the tap water, then fill with a couple of gallons of distilled water and pump out as much of that as possible, then drain the last bit. I do that to try flushing as much of the tap water as possible. At that point I fill with actual coolant. Distilled water isn't bad for warm climates, but doesn't have anything to prevent algae and sludge from developing so it will get nasty and need to be replaced every 6-12 months depending on how much you use the welder. More use is actually better than sitting.
The coolant I've been using is much less expensive than the Miller stuff, but seems to be the same ingredients and level of protection. At $25 a gallon on Amazon I consider it cheap insurance. For warm climates a 50/50 mix of this and distilled water would be a good compromise. Change it every year or two and you aren't likely to have issues. After going through so many coolers I'm convinced lack of coolant changes, and improper coolant are the two biggest causes of problems. The worst I've seen had either straight water or automotive green coolant in them....talk about a nasty cleanup job!
These are the screens I get for the pump:
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This is the Miller flow indicator from the Coolmate 3.5 and I think others. It's available from other sources for less money (like Millerserviceparts.com) but they don't have pictures like this one does.
www.mcmaster.com
Sometimes the filters get clogged , other times they just need a good cleaning up with new filter material installed . We run the 4 gallon models .
