CircleCi randomly failing specs on polymer.js app

I have app that uses Polymer.js, I'm running my tests on CircleCi. Here is how my setup looks like:

general:
  artifacts:
    - "screenshots" # relative to the build directory
dependencies:
   post:
      - wget https://selenium-release.storage.googleapis.com/2.53/selenium-server-standalone-2.53.1.jar
      - java -jar selenium-server-standalone-2.53.1.jar:
            background: true
      - npm run test:server:
          background: true
      - bower i
      - gulp serve:dist --develop:
            background: true
test:
   post:
      - gulp

Ocassionaly my tests fails. Basically some elements are not present on page: Timed out while waiting for element <login> to be present for 15000 milliseconds.

I'm using nightwatch to test my app. Always only one test fails, not always the same one. Any ideas what may causing this problem?

via Mateusz Urbański