Writing Daily isn't a Must, and if you did not get to writing on a particular day, it isn't a failure. However, there are two points to consider, one is self-honesty and one is self-dishonesty, this takes you back to your starting point of why it is you are writing, what value that is emotional/feelings/thought based of the mind did you place within writing, how have you defined writing within this process?
See, you can write every single day for twenty years, and if you aren't writing from self-honesty, as your starting point being clear, then all that writing will be for nothing, or you can write every day or second day, preferable everyday, in blogs, on journals, on cellphones, and as long as your starting point is within taking self-responsibility, learning to know yourself, change yourself, stopping patterns that isn't best for all, where the starting point is genuine, then you will not feel like a failure if you skipped a day, or could not find the time on a particular day, because you are being self-honest within what is HERE.
on the point of not knowing what to write - I would like to give you a perspective, IF we wait for things to happen to us, or things to take place within our world or within ourselves to have something to write about, our process is already within the consequential cycle of only taking responsibility for who we are within "when it is to late" - in stead of using writing to bring point HERE and to take a preventative action.
There are Billions of things to write about, and they do not all have to be Only about who you are as your character, you can write about anything within self forgiveness that exist within this world, from ALL characters, you can do writing and Self forgiveness as a Mother, because this empowers you to stand one and equal as all characters as yourself. so, when we wait for things to come up to write about, or do not see anything to write about - YET we haven't transcended LIFE/Death - then we must know we are within the mind as seeking energy to still drive us to move, and not yet the directive principle within our own lives as the Authority moving and directing.