The key word is unanimity
EU sanctions under the Common Foreign and Security Policy are adopted by the Council unanimously. That gives each member government leverage over the final decision. A country does not need to represent a majority of the EU population to delay agreement when unanimity is required.
The proposal is examined through Council working groups and Coreper before ministers adopt the decision. This means much of the bargaining occurs before the public moment when a package is announced.
A political decision becomes an enforceable rule
Where sanctions involve asset freezes or other economic and financial measures, the CFSP decision is accompanied by a Council regulation setting the detailed legal scope. The regulation is binding within EU jurisdiction.
Implementation and enforcement are primarily responsibilities of member states, while the European Commission oversees implementation. The practical strength of sanctions therefore depends on national enforcement capacity as well as agreement in Brussels.
Why one government can matter so much
Unanimity means sanctions packages can become bargaining points in disputes that extend beyond the target country. A government able to withhold consent possesses procedural leverage. That does not guarantee it gets everything it wants, because the other 26 governments can impose political costs or negotiate around linked issues, but the veto changes the bargaining structure.