Grinder invalid feed

Been happily grinding away and using weapons. Three weapons of the same rarity was going well. Lately I have been getting invalid feeds using the combinations that worked before. I read the wiki and none the wiser. Any advice on what I am doing wrong would be welcome.

Thanks in advance.

The most likely explanation I can think of is that you are trying to grind a mission required weapon: some of them got flagged internally so that you can’t accidentally grind them. Examples include the Winged Release Nove shield and the shock sniper from a couple of the Janey Springs side quests.

If you notice that the grinder seems to be taking a long time to update after you add a weapon to it, that’s probably why.

There were some additional tweaks to the grinder recipes that prevent certain moonstone grinds, but that was back in the early spring.

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That will be it VaultHunter101. I think they were the ice weapons I got from Rocket Scientist side mission.
Thanks for the speedy reply!