What Isabella pretends to have

Okay, here is what I think the problem is. Starting with chapter six in bk 1, a few texts, This is Isabella speaking:

There is nothing I would not do for those who are really my friends. I have no notion of loving people by halves; it is not my nature. My attachments are always excessively strong. I told Captain Hunt at one of our assemblies this winter that if he was to tease me all night, I would not dance with him, unless he would allow Miss Andrews to be as beautiful as an angel. The men think us incapable of real friendship, you know, and I am determined to show them the difference.

Isabella claims to be a great example of female friendship here but on what basis does she claim it. As the highlighted bits show, it is on the basis of her strong feelings.

Or consider the conversation that Isabella and Catherine have about men's complexions. Isabella begins it with a question:

By the by, though I have thought of it a hundred times, I have always forgot to ask you what is your favourite complexion in a man. Do you like them best dark or fair?”

“I hardly know. I never much thought about it. Something between both, I think. Brown — not fair, and — and not very dark.”

“Very well, Catherine. That is exactly he. I have not forgot your description of Mr. Tilney — ‘a brown skin, with dark eyes, and rather dark hair.’ Well, my taste is different. I prefer light eyes, and as to complexion — do you know — I like a sallow better than any other. You must not betray me, if you should ever meet with one of your acquaintance answering that description.”

“Betray you! What do you mean?”

“Nay, do not distress me. I believe I have said too much. Let us drop the subject.”

Catherine, in some amazement, complied.

Isabella is falling over herself trying to make Catherine aware of her feelings for Catherine's brother here very studiously describing his complexion as her favourite as Catherine has just unconsciously described Henry Tilney's as her favourite.

A final example, for this I jump ahead to chapter 11. Catherine has missed her country walk with the Tilney's and Isabella is trying to convince her that it is the Tilney's fault:

It was amazingly shocking, to be sure; but the Tilneys were entirely to blame. Why were not they more punctual? It was dirty, indeed, but what did that signify? I am sure John and I should not have minded it. I never mind going through anything, where a friend is concerned; that is my disposition, and John is just the same; he has amazing strong feelings.

Again, she claims strong feelings as the basis of her moral behaviour. We can see how strong they aren't by how she immediately loses interest in them to comment on the hand Catherine has just been dealt.

The reason Isabella can never get Catherine to read her feelings is that she doesn't have feelings in the first place. All she has is a series of passing fancies that conveniently match up with her personal interests.

Yes, Isabella lies, exaggerates and is utterly inconstant in her friendship but the reason this keeps happening, or, rather, the reason she never makes any genuine connections, is that she has no feelings herself. When she tells Catherine that she and James have discovered their feelings are identical on all the important things, we can be certain Isabella's feelings here are conforming with those of James Morland to make him love her and no other reason. Why, because she has no feelings of her own.

She lacks feelings, sentiments or, to put it another way, sensibility. Because she has no sensibility, no response to anything, she is driven entirely by her self interest.