I’ve been practising a lot as of late (a couple of waltzes in fact that my teacher had put together into a bit of a medley), but for some reason my sound is at a stand still, and I’m not sure why. When I play, my notes can’t seem to stay in tune if I hold them…they tend to go flat despite my fingerings not moving at all. Or, they start to sound very…breathy? I don’t know how else to describe it…while I play, like I’m not quite on the string or something, so the note can’t come out clearly. These notes also sound a bit off, but when I double check fingerings, they’re fine. So I’m thinking it’s my bowing, but I’m really not sure. I’ve messed around with how I hold the bow (and I think I finally have it correct…my wrist isn’t shoved down while I play which is a good thing), and I’ve tried to find a better way to bow while playing so I’m hitting the strings better, but nothing seems to totally correct the problem. Is this just one of those things that will get better the more I play? I do still hit other strings a bit when playing…like when I’m playing an open A, I’ll hit the E…and I’ve thought that might be part of the problem. It’s driving me batty at any rate, so I’m hoping you all will have something in terms of advice. I don’t start up with lessons again until next month, so I can’t ask my teacher as of yet. Thanks everyone in advance! 
Yes. Sometimes when you have no reason to believe you should have improved or will ever improve, you suddenly have.
I elevated my A a bit with one of those little tubular sleeve things on the bridge, because my A was cutting into its bridge slot a bit too much.
Another thing relevant to A and E wanting to interfere with each other is that when you’re playing a high note on your A, you’re decreasing the difference between the height of the A relative to the E, thus increasing the likelihood of E trying to get in on the action. It may help to bow a little closer to the bridge in these instances.
disclaimer: This is just personal experience, not backed by the endorsement of any actual good players. I await their input.
I’ve been in group fiddle classes where the teacher might mention to a student that they’re a bit sharp or flat and the student replies that their finger is correctly on the tape “so how could that be?”.
The placement of the finger on the string changes with the amount of weight one applies to the string. What might be in tune with a light touch is going to be different with a heavier touch. Think about it. With a heavy touch the finger tip spreads out and covers more string. Perhaps that’s what’s happening. Your touch is getting lighter or heavier as you hold notes.
Some people mast that happening by using a lot of vibrato.
I try to maintain a consistently light touch. If I clamp down it effects my speed too.
When I back off of playing banjo and guitar and focus only on my fiddle playing for a week, my finger tip calluses start to go away.
Two strings at one time? That’s something remedied with practice too. The bridge on my fiddle is a slight bit flatter than Classical because I’ve been playing a lot of Oldtime. That’s not to say I can’t hit single strings when I want to.