French language Q&A Forum

The rule concerning agreement of "tout" in different situations confuses me.  Why isn't it written "toute à l'heure" so that tout agrees with the gender of l'heure?  Thanks.

I found this explanation in Le Figaro but it does not explain why there is no agreement: https://www.lefigaro.fr/langue-francaise/expressions-francaises/2019/01/23/37003-20190123ARTFIG00047--toute-a-l-heure-ne-faites-plus-la-faute.php 

I almost never get an answer to my questions, but I will try once more. I learned that ," J'ai mal a la tete" meant "My head hurts," or "I have a headache, (sorry but don't know the keystrokes here to get accent symbols). Now, in this exercise, it's using "faire" rather than "avoir" to express physical discomfort, but the answer concenrates only on the use of possessive adjectives, not the strange change from "avoir" to "faire." I feel frustrated and very confused. Help!