FeignClient got "503 Service Unavailable" error in SpringBoot application

Question

I have two simple services, service A is calling service B using feignclient. these two services are really simple, but I often got 503 Service Unavailable error when calling to service B. the first call is always working, but this error happens from 2nd time call. there is no special configuration, it is using default configuration. does anyone has idea how to trouble shoot this?

feign:
  httpclient:
    connection-timeout: 10000
    read-timeout: 60000
    max-connections: 100
    max-connections-per-route: 100
    enabled: true
  client:
    config:
      default:
        connect-timeout: 60000
        read-timeout: 60000
        logger-level: full
  hystrix:
    enabled: true
hystrix:
  command:
    default:
      execution:
        timeout:
          enabled: false
        isolation:
          thread:
            timeoutInMilliseconds: 10000
    others:
      execution:
        timeout:
          enabled: true
        isolation:
          thread:
            timeoutInMilliseconds: 3000