Last time was all about approaching a new genre, find it here. Having a word count goal can be a very useful tool as a writer. It may be a pain sometimes but it really is worth it.
Writing is a discipline and a word count will help you stick to your craft and improve faster as the more you write the better you get. To stop from having really short chapters, I tend to give myself a page limit. I don't let myself finish a chapter until I've hit however many pages. It used to be six, but this year I've ramped it up to ten. (Don't be alarmed, I write on A5 in Microsoft Word. I just like the look of it.) I think in words it's about 2,500-3,000 words.
To make your chapters longer, have more happen in them before you start a new one. I would suggest having bulkier chapters and less chapters in total. What you thought could be three separate chapters could just fit into one. E.g in Cinderella I wouldn't have Cinderella being told she can't go to the ball, her crying to herself and the fairy godmother's appearance in three separate chapters. I'd have it all in one. The fairy godmother's appearance could be the end, making you want to read the next chapter.
Something should always be achieved in a chapter. It has to lead on to the next thing. If nothing has happened, you need more, so perhaps it's not time to move on yet. Don't end a chapter because time is passing either. If not enough has happened, just have a paragraph break, you don't need to finish the chapter. Too many short chapters make it hard for the reader to get fully immersed in the story.
If you're really struggling to lengthen a chapter, perhaps add a long conversation or paragraph of description. This may not be the best for your story however, it may just slow it down. But you can always go back and cut things when you've finished. Sometimes it's hard to see how it should all fit until the end.
To stick to a word count you have to give yourself a goal and make yourself stick to it. I have a daily word count goal myself, this way it keeps me consistent every day (of course I can write more if the desire strikes). When you have something to aim for it's likely to give you more motivation to actually do it.
Another thing you can do is to get a writing routine. I've highly underestimated the power of this recently, but once I started writing at the same time every day, a lot more got done as I viewed my writing as 'unavoidable'. If you make it something you 'have to do' like eat or shower you're more likely to do it. Once you start writing every day it gets easier to write every day. You'll probably stop less and reach your daily word count faster. On days you struggle, don't feel like the only one. Believe me, we all have those days. There will be hundreds of writers struggling at every moment of every day so don't feel down, you should feel encouraged by the amount you have written, however small. Even if you've written just one sentence, you've written more than all those people who "would love to write". You're already winning.
In short: Achieve something in each chapter, have a word count goal make and writing a part of your routine. What're your writing goals? How often do you stick to them? Let us know in the comments.