decomp can be installed backwards, if that's what you mean. Cam can be 180 off too, needs to have the lobes pointed down when you have the 3 dots up and in line. Then there's always the question of "did you really set the timing correctly?"
Improper timing can cause starting issues as well. The decompression mechanism allows valves to open when they normally would be sealed, bleeding off compression. You'd get the same effect (valves open when they shouldn't be) if the cam is installed 180 out, if the valve lash is not properly adjusted on TDC of the compression stroke, or if the cam timing doesn't match up with the timing mark on the flywheel.
I'd re-check your timing and the decomp installation, and make sure your weights and keepers are not catching and are actually retracting the decomp pin.