When you build an unit from the Nod Airstrip, you can't build a new unit for a few seconds or it won't arrive. I don't think the same should happen with a Barracks though, at least without any modding.

You do get refunded when an unit "disappears" though, so you're not really paying for the missing units

Also, you can evade the problem entirely by building multiple Airstrips (if the built unit can't arrive to one Airstrip, it'll arrive to another).

This only happens with Nod's cheapest vehicles; the buggy and the bike. It happens because the second unit finishes producing the plane that dropped off the unit you produced before it has disappeared.
The plane is actually the factory door animation, so in other words, the second unit finishes producing before the factory door has closed again, which makes it impossible for the unit to spawn in the factory.
As Rampastring already mentioned however, the unit's price simply gets refunded to the player, so you're not losing any credits and building a second airfield should solve the problem.
TD (C&C1) had the same problem by the way, but there the production state of the unit just shows "ready" when a unit is unable to spawn from a factory and will then instantly be spawned when you click the unit (or be refunded to the player when it's still unable to spawn).