Hey @patryk_wozniczka! It might be because of security setting on the site itself. For example, for discourse (this forum!), I had to explicitly whitelist the speckle.xyz origin for embedded content (somewhere deep in the settings…).
Not sure this helps, but maybe it points you in the right direction? Do keep us up to date with your findings!
PS: Welcome, and if you want to, feel free to introduce yourself!