Main drain suction

[smokey]

I have an older inground pool. I normally adjust the return flow from the pool by turning the skimmer basket. After using a Barracuda cleaner for a year (which works off the suction in the skimmer), I no longer am able to get any water return from the main drain ... only from the skimmer. I suspect the main drain plumbing is blocked (although have no idea why). What's the best way to try to clear the main drain pipe? Can I do it through the skimmer? I understand the second hole in the bottom of the skimmer goes to the main drain.