It isn't always easy to find money to invest.

September hasn't been a good investing month.  I got conjunctivitis and missed two days of work.  I'm don't haver any sick leaves so that took a good chunk out of my salary.  It has also been a very expensive month.  More money flowing out of my pocket means less money to invest.  I haven't invested my P1000 scheduled for September.
The stock market index has been performing very well this month.  That means the mutual fund NAVPS has been going up.  That means my previous investments have appreciated considerably (on paper) but it also means less shares for my September P1000 investment.  I just remind myself that I'm in it for the long term and that it won't make much difference in 20 years.