This phenomenon is called "cold-firing". I've experienced it with both of my RF lasers - a GCC Laserpro Venus and my Epilog Mini-18. I thought it was due to failing tubes but I just got my Epilog tube recharged and I still get it from time to time. The techs at Epilog told me it is a normal thing and that there's no need for concern. I just let the job run until it starts to fire normally, then I restart the job. No need to send it again from the computer. I view it as a normal part of operation. It doesn't seem to matter if the machine has been powered on for any certain length of time or not. It happens intermittently and doesn't worry me.
On the GCC, it started happening a lot until the machine became too much of a hassle for us as it was impeding our production. So we decommissioned that machine and got the Epilog. I like the Epilog a lot.