You have BAD Heads. Seems there "Was", "IS", a bunch of bad heads produced by Aspire and or you have fake heads. The real aspire Nautilus replacement coil heads have a security code on the box, if they came in a box of 5 heads. Did you check that code on the Aspire website to see if they are genuine?
I changed out my Nautilus head about 1+ week ago and have run 1+ tanks through it (I have many clearomizers, RBA's I use all at the same time) and the head I installed still reads 1.8 ohms.
Depending on how you use any bottom coil head, IE how much and how often you hit on it, it will eventually change the e-liquid browner than it originally started out. If you Chain Vape on it the head heats up and cooks the e-liquid. If you don't wash out the tank and coil head between refills of the liquid that gets carried over to the new liquid you fill with.
I've had that happen more often on smaller, less volume, tanks, especially on the Mini tanks that go on the Cig-A-Like batteries. But then I learnt to wash out the tank and head before I refill it.
Yes I let it sit for a few minutes after the new coil is put in. It vapes great for a while. I'm not a heavy vapor--about 3 mls a day. After one tank full it's changed the color of the juice from mostly clear, to brown. Draw gets tight. This is after 1.5 mls.
The other thing is the ohm resistance starts at 1.8. an hour later it's 2.7. Another hour later it becomes 3.5. By lunch it reads 5.2. Readings off my evic.
I figure I got a bad batch.