It's a matter of simple arithmetic that Scotland's constiuencies have almost never made a difference to what government is elected to Westminster - I think two Labour governments in the last 60+ years owed their majority to Scottish seats. As
@cookiemonster said, Scotland has not recorded a majority of Tory seats in nearly 70 years, so it's entirely fair to say Scottish votes don't count towards Westminster governments. And if the politicians Scotland does elect are denied the ability to contribute in Westminster, say, by having an un-hijacked Opposition Day vote, then it's similarly fair to wonder whether Scottish votes count at all.
Can't find any concrete info, but it's often repeated on Indy social media that the SNP would not be allowed by law to field candidates outside of Scotland - but if they were, and they achieved a majority of seats across the UK, the law would also mean they were not allowed to form a government in Westminster. If that is true I don't know why it would be the law, but realistically it might be the usual Twitter b0llocks!