I don't run ads on weekends so if I want to spend $400 per month, should I raise my daily budget to $18 to get closer to $400 per month?
(I hadn't realized that turning off weekends would turn off the daily budgets for Saturday and Sunday, lowering my monthly spend.)
Also, if I still am not hitting the monthly budget, what are all the options for raising spend?
Add more keywords.
Changing your budget will only affect what you actually spend if you are hitting your budget limits.
Think of it this way: there is a finite amount of traffic available per keyword (which can certainly vary depending on whether or not you advertise on the search and content networks, the season, the time of week, what Oprah is talking about on her show, etc, etc.) Likewise, there is a finite amount of traffic available for any group of keywords in an ad group - and if you are basically getting all that traffic already, simply increasing your budget will not get you more.
To LifeinAsia's suggestion that you 'add more keywords' I'd add that you want to selectively and judiciously add more keywords. Simply adding more random keywords for the sake of getting more traffic is not a good thing.
Almost certainly, what you really want is more traffic that is also targeted and likely to convert. Towards this end, you'll want to add more keywords which are carefully selected and highly targeted to whatever it is you are advertising in that particular ad group.
AWA