If you click through to a specific itinerary, you will see the fare breakdown. Does that use the AMS-BKK INCEU60 fare?

If so, it's possible that you may have got it as low as it's going to go. If you're adding stopovers at an additional fare of €100 each for the first two, and then further stopovers at €200 each, you could find the fare creeping up much faster than you expected.

If it's using a different fare basis, then it's possible that for some reason your desired itinerary doesn't satisfy some of the rules of the INCEU60 fare. But debugging that requires knowledge of all the detail of what you're trying to do.

Don't forget that travel agents may be offering you negotiated/net/consolidator fares that will not price on ITA. The only fares you'll see on ITA are published fares.