startup applications - How do I debug a program which I set to start at login, but it doesn't?

I have a program I want to run when I login. I added it using the "Startup Applications" program, as I have done before to run other things.

But this program doesn't start automatically.

If I run it manually after logging in, it works fine.

So.. how do I debug this?

The program in question is gtk-redshift, FWIW.