if i was getting dirt on rail hunting, i would not use rail lube.
you are only shooting 1 shot.
if i had problem as you say and wanted to use LUBE, i would use BREAKFREE CLP.it has teflon in it that drys to gray film.

i use it in my beretta shotgun on piston so dust does not foul it from the powder .

i know very little about crossbow and learning here as well, so take my advice as only what MAY work without any info on if it does.