I ask this in all respect because there are people who end up not having a romantic partner or spouse in their life. Yet, there is an assertion that HaShem has designated for each individual a soulmate. Does any rabbinic source have an explanation for this phenomenon? (Please cite quotations, if any, thanks in advance).
We do say that Hashem finds a match for everyone, as in Maseches Sotah 2A. Skip the next paragraph if you already know that Gemarah.
Here's a simple summary of the relevant part of that Gemarah. There appears to be an argument between Reish Lakish and R' Yochanan regarding whether a person finds a match depending on their merits. In support of R' Yochanan's view that it is independent of one's actions, the Gemarah quotes Rav saying that three things are determined for a person 40 days before their fetal conception, and one of those things is בת פלוני לפלוני, apparently deciding their match. The Gemarah concludes that in reality there never was an argument, but rather one's first match is predetermined while the second is dependent on one's actions.
The interpretation of the Meiri on this section answers your question, I believe. I'll paraphrase the relevant section in English. I got it from alhatorah.org, and I suggest reading it in his words (if you use the link I posted it will be in the sixth paragraph).
When the Gemarah differentiates between the first and second matches, saying that the first is predetermined, it does not mean one's first spouse is predetermined, and that if he (for example) divorces her, his chance of finding a second wife is dependent on his actions. Instead, the first proclamation is predetermined, while the second is dependent on one's actions. Why? Because the principle that it depends on one's actions is always true, just that before one's conception, one is deemed a tzaddik, having never sinned, and therefore they are automatically found righteous and deserving.
It follows from this reading that even finding one's first basheret can be diverted, G-d forbid. Further, the Meiri clarifies that when it says "the match depends on one's actions", that does not only mean finding the match, but also which type of person is found. Positive traits will engender the merit/ability to find a match in someone with similar positive traits.
So basically, if you remain a tzaddik then yes, it's predetermined, but otherwise there can be obstructions (G-d forbid) or diversions to people of variable quality.
Note that there are other interpretations.