Depending on how long you didn't clean it, the impellers can build up gunk or Calcium between the moving blade rotor and the middle shaft. When that happens, they will lock up and seem to be a single unit. At that point, a vinegar dip isn't enough. The acid can't get through to the buildup. I've even tried full strength HCL (muratic) but it just can't access it.
The solution is to mechanically break the gunk by twisting the shaft and rotor apart. This can be very difficult. There are special tools for doing this since they're both ceramic and there are notches for properly gripping them. Unfortunately, I have no such tools... so, I've used lock pliers and table vice grips with rubber bands for surface grab without shattering and slowly, eventually, I can get them to free up. Haven't cracked one yet but it is difficult.
Then acid bath. Sometimes through, the gunk was sharp (like sand) and it can actually wear away the interfaces between the surfaces. In those cases, I've used fine grit sand paper to smooth them out. The downside is that this makes the gap tolerance larger meaning that the likelihood of another event gets higher.
Why all this hassle? These pumps should be removed and cleaned every 3-6 months. When I didn't do it, I got a lock up. As long as I follow my maintenance schedule, no issues... I keep a spare of almost every kind I have and I swap them in to maintain the cleaning schedule. It's part of tank maintenance in my book. Why bother? Because the German pumps that cost 3x to 10x are a ripoff in my view. I'll pay with a little preventative maintenance.
Out of the nearly 16 pumps & powerheads I have and use, two have had an electrical issue but those are from my earliest usage ~ 4 years old.
Doing the math, I'm still way ahead on cost vs benefit.
What would make the pumps really better would be if new impellers were available for sale at a good price or if the cleaning tools were sold to the public. I've only found a couple of impeller types available.
