I have owned 2 Jabirus from new over the past 10 years and always done my own maintenance.
I used to apply a nickel anti seize to plug threads, but had trouble with galling.
The factory advised I should use a copper anti seize, not nickel. That seemed to fix the problem until recently.
The last couple of changes (I fit new plugs every 50 hours) several plugs have been tight to remove (binding threads) and I had to use a spanner to the last thread. On the first occasion I was removing plugs that had been installed by the factory at the conclusion of a zero-time rebuild.
So it would appear that the factory anti seize doesn't work very well.
The next change (the most recent) was even worse.
Any idea why some would have such tight threads, others OK.
WHAT THREAD ANTISEIZE DOES THE FACTORY USE?
We've used NULON L90 on plugs for a while now. It's not unusual for those threads to be a bit tight, it only gets to be a problem in severe cases.
If the plug seal is at all iffy - which can happen, particularly if there's a CHT sensor on the plug and it's not quite centred - there can be issues with combustion deposits finding their way into the thread. It's generally worse when using AVGAS and if the engine is burning oil for whatever reason (a new/rebuilt engine will naturally burn a little oil).
Some of those deposits will stick to the plug and so go away when you fit new plugs but some can stick in the threads in the head. If you find the new plugs are tight as you fit them, then depending on a few factors, it might be worth cleaning the threads in your engine... being careful not to drop crud or metal shavings into the cylinder of course!