There is no custom field ids from api call

I have problem with downloading custom fields from projects and tasks,

I have about 1500 tasks and projects on account which i want to download these from. There are many projects which should have custom fields ids but when i make api call in response there is all information except custom fields. This happens to many projects and there is no error. I have no idea what can cause those problems because there are some tasks and projects from which response is correct, but still most of them are wrong (without custom fields).

I really appreciate any help you can provide.

Paweł