We normally use "quizá(s)" with the subjunctive, mainly when we want to express possibility, but, sometimes we can use it with the indicative when we it shows a "concessive value".
"Quizás lo he escrito mal, pero es que no soy un experto"- valor concesivo
"Quizá llueva mañana"-posibilidad
Maybe it's difficult for a foreigner to understand these two values, I think it's also difficult for us!!!
Hi, A New Reference Grammar of Modern Spanish, (which is an excellent book by the way) says that quizá(s) basically means "perhaps." With this word, when the event referred to is happening in the present or happened in the past, use of the subjunctive is optional. The subjunctive makes the possibility rather weaker:
Tal vez tengamos algo de culpa nosotros mismos = Perhaps we're partly to blame ourselves (using tengamos here is common, to use tenemos would imply that the speaker definitely considers "us" guilty and may have reasons that we are guilty already in mind, and is probably ready to immediately state those reasons following the initial statement "Tal vez tenemos ........")
If th event is still in the future, the present subjunctive, or much less commonly, the future indicative is used, but not the present indicative: