Answer: Partly bad luck, partly not
I think there's a decent argument that Burns's loss in South Carolina was a bit of bad luck. Neither Burns nor Biggs had held elected office before, and neither had significant backing from the state party generally, so that one may have been a bit of a toss-up. As Geoffrey mentioned, Burns had some baggage, so given two unknown candidates, that might have offset the benefit of the Trump endorsement.
But in the Utah Senate race and Colorado's 5th, Trump chose to back outsider-ish candidates rather than those who were supported by other Republican institutions or the party apparatus. Dave Williams was controversial from the start, running as a candidate while maintaining his position as party chair, which many saw as a conflict of interest. And neither Utah nor Colorado have ever been states that were particularly friendly to Trump's bombastic style, so his choice of outspoken MAGA candidates in those races may not have persuaded voters to those candidates.
—Mary Radcliffe, 538