Karloff is dumb (and needs help with breeding)

[Tad Carlucci]

You've been line breeding, putting sibling to sibling. This can never produce a provably 'pure' line.

Looking at the new box from Catherine and Nicholas we see:

1) Proof that Odyssey Boo Boo is recessive to Foxie (already known, but it saves us having to check the charts), therefore

2) Both Catherine and Nicholas hide either Odyssey Boo Boo, or something even more recessive. One of them (we don't know which, or it may be both) hide Odyssey Boo Boo specifically.

Flipping to Nicholas' pedigree (which is also Catherine's) we see both are offspring of Isabella and Phineas. Phineas shows Odyssey Boo Boo. So we know where Odyssey Boo Boo came from.

The odds for Catherine and Nicholas offspring ears are:

Foxie hiding Foxie, or Odyssey Boo Boo, or something more recessive than Odyssey Boo Boo: 3-in-4
Odyssey Boo Boo hiding Odyssey Boo Boo or something more recessive: 1-in-4

Here we see the 3-in-4. But notice we're referring to the shown/dominant not the recessive. That your box shows Odyssey Boo Boo was simply less likely. Call it poor luck. Trying again should (we expect) produce a Foxie (remember that luck can be poor again).

If you want a 'pure' line, your only choice is to break up Catherine and Nicholas and start two lines from them. Your primary goal would be to avoid losing the traits you want. Your secondary goal is to eliminate the Odyssey Boo Boo you placed into both lines by using Phineas.

As Ivy said, you want to use something more dominant than Foxie (the more dominant the better, Genesis would be good, preferably hiding something also dominant to Foxie, or at least recessive to Foxie but dominant to Odyssey Boo). And, yes, as she said, it will take many boxes across several generations.