I'm trying to make a very low frequency timer. I've seen from experience that the best way to do this is with high resistance and low capacitance because large capacitors leak. Just yesterday I was able to configure timers with periods of around one minute, however when either the R1 or R2 values reach a certain point, the output of the timer stays high indefinitely. I've taken the large resistors out and put in smaller ones and the timer has worked perfectly. Is there any particularly obvious (or maybe not so obvious) reason for this problem?