it's f/8-f/10 and only 1/100th for shutter speed and you put 400speed film in it on a sunny day I assume. You have to use the right speed film for your lighting conditions with these kinds of box cameras. The aperture lever on a holga does nothing so you're not getting f/16 out of it either.
You need to begin by using a 100-125 speed film. This is two stops slower than HP5+. Many people find this works in all but the brightest situations.
Developer can also have an effect on film speed.I don't know what you normally use, but you might try D-76 or even D-23. Of course reducing development time will reduce highlights density, but not shadow density.
You can also tape a small piece of neutral density gel behind the lens to absorb from 1/3 to 10 stops of light.
When shooting with my Holga I usually use a 200 ISO film on most sunny days, and 400 when it's overcast or late in the day. 200 is nice & flexible for daylight, since if it's really bright out I can pull it one stop with few problems, and push it similarly as well if the situation warrants it. I usually use Arista EDU Ultra 200 (a.k.a. Fomapan 200).
I think the reason why some people use 400 speed film in a holga is that they can then put a ND filter on it for bright days or push it a few stops for dimmer light. I know I don't get the bulk film discount very often but I'd rather have the right ISO film for the job than mess around with ND filters on a bright day and I'd only push process something like delta3200 if i was in a very dim situation.