I suck at analysing paragraphs

Since year 7 I am unable to construct a decent-sized analytical essay in english tests. I only manage one and a half paragraphs while everyone else writes pages and pages.

Thing is when I write theories on games and tv shows I can go on and on about specific points, back them up with sufficient evidence and refer back to points successfully. So why can't I do that in class?!

Is it a matter of whether I like something or not that affects my performance? if so how do I fix myself to actually do the work to a great standard?

all replies appreciated. ty

Sounds to me like you are stuck for ideas. What topics are you having trouble writing about?

It's MEAL paragraphs basically. if you asked me to write one on the spot about To Kill a Mockingbird I probably wouldn't be able to.
I can write those kinds of paragraphs when it comes to stuff not related to school but in class, it's virtually impossible.

I'm currently covering Merchant of Venice; Shylock specifically.