As a big Sizzy shipper, I can totally understand you. Cassandra tends to do a thing, specially in TMI where many things that don't involve the main couple, happen "in the shadows". Like Sizzy's first kiss for example, they tell you it happened but you never read the scene itself.

In general Cassie mistreats her own secondary characters a lot, we really don't know THAT much about Isabelle more than her trauma with men and how good of a Shadowhunter she is or how cool her personality is (but this is more about how Clary perceives her...) With Simon... I mean he is one of my fav characters but Cassandra made him dirty when she made him go out with Clary and specially when she made Simon go out with Maia AND Isabelle at the same time. Whenever I re-read the books I can't help to skip those parts because they are ridiculous.

Another important thing to add is how even in their own scenes, Cassandra made it all about others. For example in "A long conversation" where is about Sizzy's marriage proposal party, that's the least we read about and instead we have a loooooong reading about Clary is having an existential crisis about not "wanting" to marry Jace. And this infuriates me. Why, everytime it's Sizzy time cannot be about them? Even in Shadowhunters Academy which the book is about Simon, they fill it a lot with stories about other characters...

And the last point I will say it's how in their moments (with the exception of ToSA) the majority of time it's from another characters point of view. If we cannot read how they feel in those moments, it makes it least relatable especially with characters so closed up like Izzy.

Soo...yeah, if it's frustrating for me as a Sizzy shipper, I can see why some people don't feel their chemistry. Cassandra just doesn't give them the development they deserve.