There are lots of reasons why this may happen, and not all are related to faulty components. Here's a technique you can try, if you haven't used this one already:
Most classic vocal mic techniques from before the 90s position the mic so the singer is singing slightly over or under the capsule of an LDC, and about 8"-12" away. Generally the "slight over or under" means that the mic capsule is above or below the corresponding lip by about one-and-a-half to two inches. The exceptions to this rule used to be only for particularly soft or intimate vocal sounds, where a singer would be positioned in the "batter's box" of the mic. The "batter's box" technique has become very popular since the late 80s/early 90s for all styles, due in part to a desire for a more aggressive vocal sound and the familiarity of many singers with dynamic mics where the on-capsule sound is typically more desirable. Also, the recent move to smaller studio spaces increases the difficulty with environmental noise which compromises the extra gain needed to use the classic technique.
The classic technique has a simple principle. The sound of a singer largely does not come out of the mouth, but off the chest and from the skull just above the nose. The sounds that come out of the mouth are lip noise, saliva noise, articulation sounds and some "airy" top end. There is also a great deal of wind and moisture that comes out of the mouth which compromises the sound (the wind creates low-end rumble at the capsule, necessitating a high-pass filter) and damages the mic (over time humidity will cause a capsule to become noisy or dark-sounding).
So, we choose to have the singer sing over or under the capsule. The choice of over or under is not at all arbitrary, but crucial to the sound you are trying to get. When positioning a mic over a singer (so they are singing under the capsule), hanging it or booming it in from above yields a bright, forward upper midrange and can really help a singer cut through the mix. The singer is encouraged by this technique to open up to the room and sing out, and favors more full-voice vocals. The nose-sound can help a singer with an otherwise soft voice nail that up-front pop-vocal sound.
Bringing the mic in from below, so the singer is singing over the capsule, has the effect of thickening, warming and solidifying the vocal. Singers with thin voices benefit from this technique greatly. The sound coming off of the chest is rich and textured, and since this technique naturally ducks the lip noise and sibilance, you can add top end and suddenly get an amazing, thick-sounding vocal with a beautiful, airy top end while everyone else is editing out lip noise and using de-essers. When using this technique, angle the mic slightly towards the sternum of the singer to exaggerate the effect, or tilt it away to minimize. The same can be said of the boomed-in-from-above technique. Also, without the extra transients of the bright lip noise and low-end wind noise, you can dig deeper on your compressor without sounding "compressor-y." This is key to getting a nice, smoothly compressed vocal without layering compressors and limiters.
I've attached two photos of Ella Fitgzgerald, you'll notice that in both she's using a small diaphragm mic, and in one it seems the small diaphragm has a windscreen for the close-up feed to the house, and a 47 is being used to capture the recorded sound using the classic technique. I've also attached photos of Sinatra and Billie Holiday. Sinatra is using the classic technique with an RCA 44, very common for him, and Billie is on what may be an Altec Birdcage or another ribbon/dynamic combo. Most vocals captured in a studio benefit from this technique, regardless of mic topology! Looking at the photo of the SDC on Ella, you may want to try our small diaphragm ML-2 when it's released, it has a model of a cardioid version of the Royer 121 as well as many SDC and dynamic mics. If you continue to experience issues, please submit a request by clicking here so we can assist asap.
Hope this helps!