If possible can you send a screen shot on how you have the alert conditions set? It might be as simple as setting the conditions where all nodes must be equal to various nodes at the same time.
The image above is a sample alert I setup so that it will only trigger CPU load for any device in a grouping I care about. If you notice it says Trigger Alert when "all" of the following apply. If you have a list of node names/ids or specific groupings then theres your problem. Based on that logic orion will be looking for a devices with all the names/ids or specific groupings at the same time. If you want to do that you need to add another condition group and set that for any under the Trigger Alert when "all" of the following apply.
The image above is how I further refine what devices I look for CPU load. In this example I set it that it must be greather than or equal to 75 and be either in the NIS group or ping response time of 53. Please note that I quickly generated this for this example. The Trigger Alert when "any" of the following apply can be used for multiple groupings you create like machine type and location.