Weekly Challenges, in itself, is challenging because the photo has to be taken in the week of the Challenge. That means, as in my case, if I don't like the Challenge or topic, I don't participate.
I don't think it is the number of images, just that you submit an image.
I run a Monthly Challenge in another forum I frequent a lot and the participation stems from the topic. B&W is hugely popular, just mentioning it brings a flood of participation. One of the most popular challenges was simply posting any B&W image.
Weekly is tough on who ever runs it because you are judging the images. I don't judge the images. Participation is the reward and comments from other members. I may change that and see if people like a Challenge where the images are judged. No one in the last year has suggested it.
My suggestion would be as follows:
1) have two people running the Challenges, one in charge and one for opinion and backup
2) Create a list of topics for 6 months ahead
3) Go to monthly challenges
4) Don't change the Judging format, I actually like that, where you declare a winner
5) I would only allow ONE IMAGE submitted, the best one
6) Have a 3 week period to submit and 1 week period to vote to make up the month
- so you need a submission period, 1st of month to 23rd, 7 days to VOTE and declare the winner on the 1st of the next month and so on.
Hope this helps.