This is a perpetual problem I have and I find it quite annoying. Yes, I know its kinda subjective, but that doesn't change the fact that, to me at least, my conlangs are always hideous.
For a recent example, my most recent conlang has agglutinating pronouns. For instance, the word for 'I' is 'ta' and the word for 'we' is 'tai' (pronounced like 'tie', minus the aspiration of course). That's not too bad. The issue is when I add the nominative marker -s: 'tas' and 'tais'. To me, 'tais' just sounds hideous.
In past I think it was because I used the voiceless velar fricative far too often. But even if I don't include the phoneme my language sounds hideous, and artificial to the point that its sounds robotic. Though the latter is probably because I have a hard time NOT making conlangs 'logical'.
There are of course nice-sounding conlangs out there, and even natlangs, but I just can't seem to imitate the sound of any other language for some reason, and I don't get why. And I specialize in phonology, so I don't get why I can't get the sound right. Even if my conlang has the exact same phonemes, allophony, intonation, and phonotactics as a natlang, it will sound completely different and I can't figure out why. I mean, how can I screw up the latter? What am I missing?