Estou executando um aplicativo Rails 3 com delayed_job. O problema que me deparei é que, embora o aplicativo esteja adicionando trabalhos corretamente à fila, eles nunca estão sendo processado

Minha class

class User < ActiveRecord::Base
  after_create :send_welcome_email

  private

    def send_welcome_email
      UserMailer.delay.welcome_email(self)
    end
end

Inspecionando as coisas através do console do Rails, posso ver que há trabalhos na fila. Também posso ver que houve 0 tentativas para executar os trabalhos. Girar um trabalhador Heroku não faz com que os trabalhos sejam processado

Alguma ideia

Obrigado

Edit: Tentando limpar a fila de trabalhos, como sugerido abaixo, executeirake jobs:clear e recebeu o seguinte erro

rake aborted!
uninitialized constant Rake::DSL
/app/.bundle/gems/ruby/1.9.1/gems/rake-0.9.0/lib/rake/tasklib.rb:8:in `<class:TaskLib>'
/app/.bundle/gems/ruby/1.9.1/gems/rake-0.9.0/lib/rake/tasklib.rb:6:in `<module:Rake>'
/app/.bundle/gems/ruby/1.9.1/gems/rake-0.9.0/lib/rake/tasklib.rb:3:in `<top (required)>'
/app/.bundle/gems/ruby/1.9.1/gems/rake-0.9.0/lib/rake/rdoctask.rb:20:in `<top (required)>'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/tasks/documentation.rake:1:in `<top (requ
ired)>'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/tasks.rb:15:in `block in <top (required)>
'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/tasks.rb:6:in `each'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/tasks.rb:6:in `<top (required)>'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/application.rb:214:in `initialize_tasks'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/application.rb:139:in `load_tasks'
/app/.bundle/gems/ruby/1.9.1/gems/railties-3.0.7/lib/rails/application.rb:77:in `method_missing'
/app/Rakefile:7:in `<top (required)>'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:2373:in `load'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:2373:in `raw_load_rakefile'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:2007:in `block in load_rakefile'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:2058:in `standard_exception_handling'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:2006:in `load_rakefile'
/usr/ruby1.9.2/lib/ruby/1.9.1/rake.rb:1991:in `run'
/usr/ruby1.9.2/bin/rake:31:in `<main>'