Sometimes some notes are muted when I play them. My fingers are all in the right place, and they're all touching the strings (not even pressing down).
My sister plays the guitar and she tried to help, but she couldn't.
I'm unsure what I'm doing wrong, please help.
Try pressing down. You need enough pressure to make sure the string is firmly pressed against the fret in front of your finger (but not so much you bend the note out of tune). Also make sure your other fingers aren't touching the string accidentally or that will mute it.
It can be a number of things. As others have said make sure your other fingers are not touching any strings. Make sure you fretting finger has ONLY touched the intended string. Touching the string above or below will mute them. Make sure you are pressing down JUST behind the fret wire, if your finger is on top of the fret wire it will deaden the sound. One last thing......and I am quilty of this is forgetting to roll up my sleeve on my strumming arm and having it lay on the strings which deaden the notes. Make sure your strumming hand is not brushing the strings.This is how we purposely "palm mute" or chunk a strum.
What type of uke is it....make model and size. Do you know what type of strings are on it. I am going to be blunt here but some cheap ukes especially sopranos with no name strings have a lot of dull dead notes and can sound like crap all over the fret board.
If you are just starting to play, having some arch in the fingers (unless you are barre-ing a chord) is essential, as well as having the thumb somewhere on the neck where you provide support against your fingers.
I have my students start off by making an "OK" sign, and moving the ukulele in-between the fingers.
As the others have said, make sure you are behind the fret (ideally as close to the fret without being on top of it), and make sure your fingers are pressed down hard enough that the strings are vibrating only between the fret and the saddle.
All that said, I have heard plenty of very good players unintentionally mute a string here and there.
If you're just starting out, it's very easy to misplace or not press on the string correctly. You need to curl you finger over and press with the tip. The Bb problem Nickie mentioned is one I had and still have as you are pressing two strings with one finger and you need to ensure your fingers are placed so they press both strings evenly.
Half the secret is to take your time fingering the chord until you can do it reliably before trying to speed up to the desired tempo. (Advice I regularly fail to take I'm afraid)
I problem I found when starting out was I was worried I couldn't get to the next chord on time so I was always trying to rush the chord changes. Once I realised that I actually had more time than I had realised, I started to relax and my chord changes improved and became much cleaner.