About node_modules after I created zat new

When I created new app or after zat new to make new app, and run the app to zat server in iframe.html is working. After this I install, "npm install" to make have node_modules because I want to use some zendesk-garden-tools, etc. I try again to run the app, and it is not working. I think the problem is the node_modules is not working after I install "npm install" what is the problem of this? and how to fix this problem? Thankyou in advance!