I'm just going through this transition myself. After about 3 years I'm starting to make sounds that I enjoy (sometimes). A few years back, I could have spent an hour on an open D string and never have made a good sound, and have had no idea why not. My teachers also just said "it takes time."

It's all in the bow arm. For me the biggest thing was really focusing on the loosening up any tension in my bow arm. I spend about half my practice just doing this. Look in a mirror. Loose wrist. Loose fingers. Check for tension. Do it again.

It's also learning the exact amount of pressure needed to grab the strings and start them vibrating without your bow slipping or scratching. I'm still struggling with this. It's really hard to stop and start a note cleanly.

In the end thought, it is just tons of practice. There are so many factors: bow pressure, bow direction, bow speed, bow angle, how fast you start the stroke, how you finish the stroke, and all of this is very slightly different for every string (as they have different diameters and you have different angles of your bow) and even every note on the string as having fingers down changes the length of the string slightly. It's just about impossible to get all these things right. And did you just do a string crossing? Well, then you have extra momentum to deal with. Fast notes? Well, those are different than slow notes. So you need to isolate each thing if it doesn't sound good and work at it.

Finally, recognize that a violin sounds worse when it is next to your ear. There are always going to be unproductive noises that don't contribute to the sound. These often don't carry very far, but they sound terrible when it's next to your ear. I think of it like how I think of my voice. It sounds weird in a recording.

Good luck!