can't seem to apply tweaks on tweaks

[Kieren MacMillan]

Hello all,

I don’t quite understand when a tweak will apply and when it won’t…

For example, I have an ottavation function for “one-off” octave pops:

octU_single = {
  \once \override Staff.OttavaBracket.stencil = #ly:line-spanner::print
  \once \override Staff.OttavaBracket.bound-details =
    #`((left . ((X . -1) (Y . 0) (padding . -0.5) (stencil-align-dir-y . 
,0.85)))
       (right . ((X . 2) (Y . 0) (padding . -1.5) (text . 
,(make-draw-line-markup (cons 0 -1))))))
  \once \override Staff.OttavaBracket.font-shape = #'roman
  \once \override Staff.OttavaBracket.bound-details.left.text = \eightva
  \once \override Staff.OttavaBracket.left-bound-info =
     #ly:line-spanner::calc-left-bound-info-and-text
  \once \override Staff.OttavaBracket.right-bound-info =
     #ly:line-spanner::calc-right-bound-info
  \ottava #1
}

Once this is applied, if I try to add a tweak later (e.g., changing 
#’shorten-pair, either “inline” [in the content code] or “externally” [via the 
edition-engraver]), it doesn’t seem to apply.***  But I swear I’ve had [many 
many] other situations where I _can_ apply a tweak to a previously-tweaked 
object.

Can anyone enlighten me as to why this is true?
And, hopefully, how to work around this issue (i.e., how to apply tweaks on 
tweaks)?

Thanks,
Kieren.

*** No full snippet included, because I’m only looking for 
explanations/documentation/links/comments at the moment.  =)
________________________________

Kieren MacMillan, composer
‣ website: www.kierenmacmillan.info
‣ email: address@hidden