Why was the Senate unsuccesfull in regaining control of the republic?

Since the US Senate has never had control of the modern day Republic, I assume you're asking about Roman History.

Cesar was very wealthy and adopted young Octavius in his will. Octavius was thereafter seen as Cesar's heir apparent and used his inheritance to attract Cesar's army to join up with him. Octavius soon gained a level of control equal to Cesar's, beyond the Senate's ability to deal with.