I created custom post-types in my site but these posts are not shown in the RSS. Only the regular posts appear there.
What could be preventing them from showing up there
They don't normally show there That is how they are supposed to work. Each CPT has a feed of it's own by default Everything in WP has a feed it seems!
But if you want them in your main feed
this can go in your functions.php
You see this line:
That includes normal posts and my CPT of ve_products in my main feed
You would swap out that for your CPT, if you have more CPTs, add them into that array