This is the sort of thing it's hard to give a grand historical narrative on -- the likes and dislikes of critics have never been monolithic. In the case of The Empire Strikes Back, the bloc of haters wasn't even necessarily large; David Gerrold (famous for The Trouble with Tribbles and the Chtorr books) wrote in a two page spread in Starlog #38 that

Just about every other critic has been telling you how good the picture is; they've been falling over themselves to tell you. It's embarrassing. I feel guilty for not liking it as much as I'm supposed to.

His personal quibbles are partially with the science fiction aspect; the Millennium Falcon's repairs on the "asteroid" done without spacesuits, the weird ecology, the inaccurate ship movements. He also notes

All the chasing and racing is very exciting, but it doesn't seem to have a larger purpose.

I think there's an important point embedded in here, but let me hit a few more contemporary reviews:

What we have, in fact, with The Empire Strikes Back is a long, rather otiose and decidedly open-ended story which starts with Darth Vader sending out the forces of the evil Empire to flush out the good Rebels from their sanctuary on the ice planet Hoth.

-- from The Guardian

"The Empire Strikes Back" has no plot structure, no character studies let alone character development, no emotional or philosophical point to make.

-- from The Washington Times

Mainly it's marking time: the characters take a definite backseat to the special effects, and much of the action seems gratuitous, leading nowhere.

-- from Chicago Reader

The critics all have their quirks (Gerrold is the only I've seen, for instance, who complains about unrealistic gravity) but a common thread seems to be the formlessness of the plot. The original Star Wars was entirely self-contained; this one sets a number of threads that we know, in retrospect, have a resolution, but in 1980, before any sense of what Jedi would contain, seemed at best cliffhangers, and at worst forgotten elements.

For example, Han Solo's friend ​Lando Calrissian famously "just made a deal that will keep the Empire out of here forever" (video clip here). He then is responsible for trying to help get Han back, but fails, and in terms of a character arc (presuming it stops there) is kind of miserable: a person who realizes their mistake, turns it around in a way that appears purely selfish ("I have altered the deal, pray I do not alter it further") and fails to redeem himself. This of course sets up for glory in Jedi but for a critic who is watching the film for the first time in 1980, the story does come off as incomplete.

This makes it relatively straightforward to say what changed to tilt some critics the other way (but not everyone!): Return of the Jedi came out to resolve and make the incomplete plot feel whole. It should also be emphasized that not every critic was familiar with science fiction enough to grasp the complicated elements -- just like I discussed with The Thing where some top reviewers had their first experience with gruesome body horror -- not everyone had the experience and vocabulary to grasp the movie on a first viewing. This is made most stark in a positive review, one from The Hollywood Reporter. The review does complain about plot pacing just like the others ("...in the final third of the film when our doughty band of rebels divides up and the story rather awkwardly shifts between three different locations...") but most notably makes a mistake, which I'll boldface in case you miss it:

As to the present episode, despite a couple of phoned in messages from Ben Kenobi (Alec Guinness) which, collected, must last all of 40 seconds, most of Luke Skywalker’s good advice comes from a fascinating green-hued character named Boba Fett (Jeremy Bulloch) — and I’m still not certain whether he’s a skillfully animated puppet or a real live human being. Whichever, he’s one of Lucas’ strongest assets in this Star Wars incarnation.

Yes, Boba Fett, dispenser of Jedi wisdom. Both Yoda and Boba Fett were new characters, but more importantly, there had been only one prior outing in the Star Wars universe, so some critics hadn't fully adjusted to Lucas's brand of sci-fi.

...

Kapell, Matthew, and John Shelton Lawrence, eds. Finding the force of the Star wars franchise: fans, merchandise, & critics. Vol. 14. Peter Lang, 2006. (Chapter 16 is about the reception from both popular and scholarly critics.)