Ideally, things would work great, but when they don't here are some things you can do to alleviate some of the stress.
Be flexible. So the computer lab is booked this week. Can you use laptops? Rotate the students around the computers you have in your room? Change your computer lab time so that students have double access on another week. No matter what you decide, you have to find a way to be flexible with communicating outside of the classroom.
Manage Scheduling. Some of these writing tasks or assignments may come up before your students have had exposure. Take a moment to schedule in some background and prior knowledge instruction to aid them in completing the task.
Manage Scheduling. Make sure you have a defined time to go to the computer lab for your students to send emails, but also for them to read and respond to emails. If the computer lab is not available, refer to number 1!
Written Blogging. If you're already having issues scheduling the lab time, blogging will be a little difficult. That doesn't mean it can not be done. Instead, have students keep a journal after each writing and computer session, then when the lab is available or during small group times, have students log-in and add their pre-written blog to their site.
Be prepared to go on. Sometimes you won't be able to be in contact with your ePals in between weekly assignments and that's ok! Have your students continue their research and questioning so that when communication starts back up, your student is prepared with what they want to say.
If communication ceases completely, your students can continue with the project using the information they already have, and doing independent research-- while this is not ideal, the students will still develop an understanding of the cultural and environmental differences between them. This will also make for a great reflection writing for them in the end.