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The D string is thicker than the A or E so you have to use a bit more effort to get it to vibrate. The G is even thicker I am surprised you have no issues with it too. Your D string could have gone tone deaf due to age. It is one of the most played strings, I think A then E followed by G IMO. You might not be pressing hard enough with your fingers for the notes on G. Being thicker it takes a bit more pressure., the thiner strings are more forgiving. Make sure you are using the "correct" fingering technique. I find that I have trouble getting a string to ring when switching between them quickly. Its like it takes a sec for the bow to grab hold, dunno if this is similar to your issue? Really, unless you post a vid of the problem or have a teacher look at it everything else is monday morning quarterbacking. fiddle on-pfish.
"Please play some wrong notes, so that we know that you are human" - said to Jascha Heifetz.
Welcome to Fiddlerman, Ed.
There are so many variable's that affect tone and clarity on a violin. It start's at the nut and work's all the way back to the tailpiece. I'm not going to go into the "sound post", that's another issue which has nothing to do with finger action but affects the quality of the sound coming out. (You can send a sound sample to cdennyb for a sound analysis after you get your violin in order). The violin should be setup properly for good tone and clarity. Let's begin at the nut end.
1. The nut need's to be notched / groved so there is enough clearance between string and fingerboard so that the string doesn't vibrate when played open. Too deep and you get vibration.
2. The fingerboard should not be humped up in the middle, it should be lower in the middle than at the ends. Using a 12" steel straight edge, lay the straight edge on edge from the nut to the end of the fingerboard near the bridge. The fingerboard should be lower in the center. There is more space between the fingerboard and the steel edge on the G side than on the E side. This is called the "scoop", it's put there because of string vibration, the G vibrates more than the E, so more scoop /clearance is required. I'm not going to give dimention's for clearance's here because there are different opinion's on that.
3. The radius of the fingerboard perpendicular to the string's need's to be correct.
4. Bridge height should be so you have proper spacing between string's and fingerboard at the end. The bridge need's to be cut and sanded as to acquire these dimention's. It has to fit the belly / top of the violin correctly. The string's also need to be spaced to specs. on the top of the bridge as well as at the nut.
5. There should be proper distance between the bridge and tailpiece for correct striing length, this is called string "after length". This dimention is acquired by adjusting the tail gut length under the tailpiece that loop's around the "end pin".
Don't take for granted that when you buy a new violin that all is well.
Given that everything from nut to bridge is setup correct, you shouldn't have to press harder on the G than you do on the E string. By having to apply more pressure to one string than the other's slow's down your finger speed and affect's action. There is less tension on the G than on the E so there should be no difference in pressure applied.
Now when it come's to your D string, you may be laying the pad of your finger's on the string rather than the tip, this may be causing you to touch the string next to it and not get clean down pressure on the D causing an unclear/distorted note. This may be happening if your left elbow isn't directly under your instrument. It causes what I call "short fingers", that is not being able to stretch your finger's across the fingerboard in order to reach the string with a arched finger. Finger's need to bend so the fingertips touch the string. So many people have a hard time pressing down a string with their pinky because it's hard for them to arch it because there's not much strength in the pinky finger.
And,,,, as other's have suggested, it just may be the string itself, try switching first.