Market's Lay Book% is Blank

I have wondered lately why my trigger has not fired for a bet or two over the
past 3 days. Then I realise they all had one thing in common being those particular races
Markets Lay % was blank. My trigger has a condition >97. All other races were okay.
Any ideas why ?
I could work around it but that won't solve the issue at source if indeed it can be solved at source.
Cheers
Ian

Hi
A market's lay book % would be blank if at least one selection has no lay price offered. e.g. at least one selection's back price is 1000, or indeed if no one has offered to back a selection at any price there would be no lay price for that selection.