Hi,
of course you cannot expect an even distribution. You know about Normal distribution resp. Gauss distribution?
As far as I understand: Your survey is running already.
So you cannot change anything in the structure.
What you can do:
1. Wait until you have your goal of 25 respondents in all groups.
It's usual that in some groups you will get an overquote.
Here you have the chance to remove bad quality responses (happy clicker, speedster, ...)
2. If one groups is full, change the relevance of that group to "0" and add this former relevance to the least filled group.
Example:
Initially:
Group 1: random==1
Group 2: random==2
Group 3: random=

Group 4: random==4
Now, let's say: There are 25 answers in group2, group 4 is least filled (only 21)
Group 1: random==1
Group 2: 0
Group 3: random=

Group 4: random==2 OR random==4
Meaning you have to check permanently.
Joffm