Welcome to Android Central forums.
What app are you using to add these events?
Is it adding them to the calendar on the phone just locally or does it add them to your actual Google calendar?
On the flipside of that you say it isn't adding them to one of your accounts, the one that it is adding them to, is it a Google account? Do the items show up on the actual account, like if you logged in to a PC and went to the account online, it shows up there or it only shows up locally on this calendar?
In the calendar settings is the account you want to use the default account on that calendar? I know you're saying it is the default for the device but I'm asking that you look at the calendar app and the app you're using to add these events and see if there is a way to make this missing account the default. One way to know which is the default for the calendar would be to create an event using the calendar and see which account it attempts to add the event to.
Are the accounts properly sync'd? If memory serves there are different levels to syncing calendars. There is read only and read, write and remove. One way you can share your calendar but whomever it is shared with can view but not alter and the other accepts changes.
Let us know what you find and we can offer you more advice. Please include what this other app is.