Without pictures this is a guess
Also, with the advent of the 445, we don't talk about these as much, so the info is "buried" a bit
First, the 405 requires the lens to be a bit closer than others.
I would think that you need to remove the focus ring and screw the lens in a bit farther
If the lens bottoms out, then the diode was not pressed deeply enough into the module
The 405 actually needs to be pressed BELOW/PAST the back surface of the module
This picture is actually for one of the Merideth adapters, but it shows what I am talking about
If you used the "back half of the module" pressing technique, this could be the problem
Second possibility is simply that the 405 is harder to "see"
If the above doesn't help, then have someone else shine the laser and actually go down to the wall to see how "big" the spot really is
It may just "look" big from where you are standing
As far as the "speck of dirt" is concerned, rotate the lens while the laser is on.
Does the "mark" move with the lens?
If so, the dirt or blemish in on the lens
One can rarely actually "see" dirt on the diode window
When the diode window is dirty, either the output is lower or the spot is misshapen
Peace,
dave