Cogital I think when you are following a horse you have to ask yourself how long do I follow it for?
Personally for me I use a three strikes and your out system and yes I have had them win on say the fourth or fifth time and not bet them that's the way it goes. I suppose another guide would be price IE. If u have backed it eight times and it has not won then the price you obtain should be more than 8/1 as this horse is 25/1 then I would imagine that trainerlayer hasn't bet it more than 25 times so on this occasion it's all well and good backing it, but if it was say 6/4 it would be utter madness to back it whether it wins or not. However people your betting the maths of it all has to come into play.