Six years is not a particularly long time to wait for a sequel, but in the case of Frozen it is hard not to feel that the decision to return was totally motivated by money-making potential and not much else. Of course it is naïve to think that any sequel is made out of love for the audience etc and without money in mind, but usually it doesn't feel so brazen as this; and that impression is not helped by Disney digging up its own back catalogue to remake and resell them. In the meantime Frozen made over a billion dollars, with hit records, merchandise, stage shows etc, so the sequel was not a great surprise - but it is a bit of a disappointment.
The plot is the aspect that hurts it. It feels like it was written by a committee, and puts too much effort into keeping all the characters busy by giving them something to do. As a result the plot feels convoluted due to the number of threads that don't have direct relationship to one another. In addition, the threads themselves have a lot going on that is hard to translate for children - the link to past generations, the origin of powers; all sorts of things like this rob it of impact because it feels less smooth - you can hear the gears grinding as it moves along. There are several strong songs, not quite the strength of the first film but that was a high bar to clear. There are funny moments, but the whole thing does feel less 'fun'.
At the same time my reduced investment in the film meant it had less emotional impact; it doesn't take much for me to get choked up but nothing did anything for me in this one. It wasn't awful of course, just disappointing. It still offers the songs, the laughs, the spectacle but it does it in such a heavy-set way that it doesn't spark or feel organic as it does it - and as a result the feeling gets stronger that this is only really about squeezing more money from the audience.