Macro partitioning

Acute ingestion of plant proteins generally stimulates Muscle Protein Synthesis (MPS) to a lesser extent than animal proteins when matched for protein mass, due to lower leucine content and digestibility.

If you eat a small amount of plant protein, it won't spike muscle building as much as whey. However, this doesn't mean it doesn't work; it just means you need to eat more of it to get the same 'signal' to build muscle.