As the interplanar energy has not coursed through their brain on a failure, I believe they can retry until they get it right. =
That's my interpretation too.
I was actualy looking for replies from a rules p.o.v but I agree with you two.
My rationale is that according to the book a spellcaster might have to beat the odds of up to 3 dice rolls (d100, d20 to hit single target and maybe a Saving Throw) and the effects of variable damage spells are dependent solely on luck (no such thing as a "critical hit" for magic rolls).
It seemed unfair to interpert the spell failure chart as removing the failed spell from memory until 6 A.M. next day. It would be like ruling a weapon automaticaly breaks on a failed attack roll
This means, according to our ruling, that the "Spell Failure" rule actualy represents the ability of a magician to unleash a spell in a less-than-perfect, chaotic or confrontational situation.
If you are casting Heal on a fallen comrade in the safety of your own home you can eschew the d100 roll altogheter we can just assume you get it right after a minute.