Q:

How to express an exception, being able to do sth. normally, but not today?

I want to express an exception and don't know which sentence would be appropriate. I want to say that I just wasn't able to do it today, but I am able to do it normally. Which of the following sentences has got this meaning; or are there better ways to say this?
(I guess some of these sentences actually mean the opposite, I am not able to do it, as always.)

いつものように出来ない
いつもみたいに出来ない
いつも通りに出来ない

Or maybe one of these sentences?

いつものようには出来ない
いつもみたいには出来ない
いつも通りには出来ない

I guess that いつも通りに(は)出来ない is alright, but I am not certain. Also, maybe these sentences in the past tense would make it easier, but I wanted to try it with the present tense.
Also, adding a 今日は to the sentences could make it easier, too, I guess.

A:

Your first set of three sentences without 「は」 used in them could mean two very different things.  This represents one of the soft spots of informal Japanese today.

1) "I am being unable do it (today) as good as I usually do."  Today is an exception.
2) "As usual, I could not do it (today)."  Today is no exception.

By adding 「は」 as you did in your second set of three sentences, the semantic ambiguity instantly disappears and the sentences can now mean only one thing:

"I am being unable to do it (today) as good as I usually do."

One can easily (and naturally) express the same idea by adding 「に[限]{かぎ}って」:

「[今日]{きょう}に[限]{かぎ}って、いつものように/いつもみたいに/いつも[通]{どお}りにできない。」

By using 「限って」, you can drop the 「は」.
(Lastly, it is officially 「できない」, not 「出来ない」.)

