SSRS - Report doesn't loads via Report Builder, while it does via SQL - sql

Question

this is my first question here.
I struggled days and days trying to find a solution everywhere with no success.
Basically I have a standard stored procedure pulling out a report dataset in a few seconds (5-6 seconds).
It aggregates (GROUPING BY and SUMMING) 23000 rows.
Indeed, my final dataset comes out with 4 rows and 33 columns executing, as said, in 5-6 seconds.
Unfortunately, while trying to load it via ReportBuilder, it loads endlessly (querying SQL Server, the StoredProcedure remains stuck in a RUNNING status forever).
Everything on ReportBuilder (DB Accesses, Dataset, Parameters, Matrix....) is right configured: I was indeed able to load it until I added a few additional (4) fields.
The SQL dataset is basically something like:
PARAMETERS DECLARATION
SELECT
FIELDS
FROM
(SELECT
FIELD A
SUMS
FROM
TABLE
JOIN TABLES
WHERE
PARAMETERS MATCHING
GROUP BY A
) AS B
ORDER BY FIELD
An "external layer" SELECT was needed to make some calculations on some FIELDS, also in some cases using some PARAMETERS.
That's it.
I use to work with huge datasets, sometimes pulling out 30,000 rows with 110 fields, but if something loads via SQL it also does always via ReportBuilder: this is the very first time it behaves in this different way.
So I'm asking if there are some strange SSRS/ReportBuilder limitations I never faced in my experience.
Any help would be really really appreciated!
Thanks in advance to everyone who'll spend time :)