The rules change more often than in most areas of Bulgarian law, and refusals usually come down to a document rather than to the substance of the application.
What this covers
Residence. Long-term residence on various grounds. Permanent and long-term resident status. Renewals, and what happens when there has been a gap.
When it is refused. Appealing a refusal to issue or renew, and appealing coercive measures and entry bans.
Family. Family reunification, and the status of family members of Bulgarian nationals.
Citizenship. Bulgarian citizenship by descent, naturalisation, and restoration of citizenship.
Documents underneath all of it. Legalisation, sworn translation and the personal number for foreigners, which most applications depend on.
When to instruct a lawyer
Before submitting, if there is anything unusual about your case. Immediately after a refusal, because a period runs there too.
Unusual means: a gap in your residence, a previous refusal, documents from a country with complicated legalisation, or grounds that do not fit neatly into the standard categories.
One thing people get wrong
They treat a refusal as final.
Some refusals come down to a missing or improperly legalised document and are resolved by a better prepared fresh application. Others should be appealed. Which of the two applies to you is visible in the reasons given for the refusal, and that is the first thing to read properly.