I know when I add a second spouse to a person you will not see that person in the tree unless you specifly click on it but it will show up when I generate a chart. So I add the second spouse & their children but I get the feeling the software may not be able to show EVERYTHING but it is there when you enter it, just hidden.
Is that what you mean, that you enter the children into the tree but they don't show up when you view it in the Tree builder?
Within my chart I genertae if I have a person with 2 spouses & children to that relationship, it will often be displayed the second spouse on the outer edge of the chart I generated NOT linked into the actualy tre itself.
For example I have 2 females with 2 spouses...the chart in the tree will hvae this displayed next to the first female (1) & a (2) next to the next female...then on the side of the chart you will find another (1) & (2) with the name of the second spouse/children to female number (1) & so on for number (2).
So check the people who have more than 1 spouse & if they have a number next to their name on the chart you generated, then look for the same number loated elsewhere on the family tree which inicadets this (1) is linked to the other (1).
Confused yet LOL...
If this does not help then I have no further knowledge/experience on why your step children are not being generated apart from remeber to SAVE your family tree in the Family Tree Builder & then PUBLISH it so your family tree is updated within the web site of MyHeritage.