Does your umbrella cover every company you own?
Only the companies it names, and only above the policies it lists underneath it. An umbrella has its own list of insureds and its own schedule of underlying policies. When a group adds a company, moves a policy to a new carrier or drops one, the umbrella does not update itself. The check is to lay the umbrella's schedule next to the group's list of companies and its current policies, and make sure all three match.
What the umbrella actually lists
An umbrella sits on top of other policies: general liability, auto, employer's liability, often liquor. Its declarations carry two lists. One is the named insureds, the companies it protects. The other is the schedule of underlying insurance, the policies it sits above and the limits it expects them to carry.
A company that is not on the first list is outside the umbrella. A policy that is not on the second list may not have an umbrella above it at all. Liquor liability is the one worth checking first, which is why it gets its own section in the liquor guide.
How it drifts
A group's umbrella is usually written once and renewed many times. In between, a new company opens a location, a policy moves to a different carrier, a store is sold. Each change is made on the policy it touches, and the umbrella keeps the old picture until someone updates it.
Umbrellas commonly carry a condition requiring the underlying policies to be kept in force at the scheduled limits. If one lapses or is reduced, the umbrella generally pays as though it were still there, and the business carries the difference itself.
The check
Three lists, side by side: every company the group owns, every policy currently in force, and the umbrella's named insureds and schedule. Any company or policy that is on the first two and not the third is the question for the agent. The same check is worth doing every time a location opens or a company is formed.
One renewal date for the group makes this easier, because everything is updated at once: Should a restaurant group have one renewal date? Whether the umbrella sits above the liquor policy is in The liquor liability gap, and the same naming problem on the main policy is here: Is a new LLC covered by my restaurant's policy?