I have been reflecting about practice and since it has been raised here I thought I'd add my thoughts.
My children started with the piano. While I wouldn't say I can play the piano I could do enough to help them with practice and piano broadly went well with practice done every day from the beginning.
Then my son took up the cornet, where we were all completely out of our depth. After about 2 or 3 years I got dragging into learning too and about 6 months after starting it I began to understand what I was doing and started to be able to practice.
We then added other instruments, flute and sax, for my daughter which didn't cause any problems but I taught myself the beginnings of playing the flute using the Abracadabra book so it wasn't that difficult, not compared to trying to learn the tenor horn anyway.
What really made me think about this was that my daughter started singing lessons nearly two years ago. I thought the reason she didn't practice singing was because she was already fully occupied practicing the piano, flute and sax. But I am now sure I was wrong. I think that she struggled to practice singing because she (and I) were not sure how to practice it. We didn't know how to do the warm up exercises properly at home and I wasn't very good at helping her sing the pieces either.
Having focused on, taken and passed the Grade 2 in December it suddenly seems that it is easy to include singing practice in our daily routine and I think this is because my daughter now understands how to practice singing.
I can't believe we are the only people who have found this to be the case.
Maybe some of the people who don't practice really don't understand how to practice.
Dora

I think you have hit the nail on the head, Dora.

I always try to incorporate into my lessons tips about practising - in the early days, how to practise a piece changing later on to "how will you practise this piece?" But thinking back to my own early lessons as a 9-year-old, I was just set a piece or several to do, without any tips about practice at all. Too often, a child will think practising entails simply playing through a piece (and uninitiated parents too, sometimes) and warm-ups don't even get a look in.

Well, to be honest I'm sure my daughter and I still don't understand how she's meant to practise singing, despite the fact that Grade 6 is on the relatively near horizon. Can you share the secret? I must ask her if her teacher has ever made any suggestions. Rather unlikely, I think. She certainly has no warm up exercises...

Our approach is to use a song she's very familiar with as a warm up. She just sings it through once or twice in a fairly relaxed way. After that she'll sing the songs she's working on two or three times. If there are passages where I can tell she's obviously struggling with the notes I will pull out my tenor recorder and play along for a few bars and she can then focus on these few bars and try and fix them. If she knows she's having problems she'll pick the notes out for herself on the piano. But I do think this is a difference between practising singing and other instruments; I think it's harder for both the singer and the non-musical audience (me) to spot the bits that need attention, especially if you're working without a recorded accompaniment. Of course, there is also the issue of diction with singing. Here I am the person that tries to understand what on earth my daughter is singing about, and I let her know if she seems to be singing complete nonsense. I think it must be extremely difficult for a singer to appraise their own diction and fix it.

In general, though, I agree that many practice problems probably boil down to ignorance of how to practise.

Incidentally, I thought from the way your post started that you were going to tell us you had decided to take up singing lessons!