I have an intermatic pool timer that will not turn on automatically, I have to turn it on manually, but it will turn off automatically. Can not figure why.

Depending on the MODEL of Intermatic time clock, it could be any number of different things.

If Mechanical, It may have dropped the "On Tripper", it attaches to the wheel, and can have several on/off times with multiple trippers. 2 is the minimum needed, at least 1 on and 1 off. If Programmable, Reprogram.