Validation error message for the mandatory field is not shown when it is leave as blank while executing the test. Instead it is showing the 'cannot locate the element' message and failed. Sometimes it is passed.
Without being able to see the test run error, this sounds like a timeout issue. The test isn't being allowed enough time for the element to exist/appear, and is causing an error to be returned before the element exists. You may attempt to remove the issue by increasing the timeout for the step. This setting will be in a couple of places depending on the step you are modifying, but will always be contained within the properties box (upper-right corner of WebUI QA edition).
If these settings being "tweaked" results in no change of the symptom and the error points to a timeout of some type, try adding in an execution delay. It is located in the Add > More section of the toolbar (see attached "execution_delay.png"). Move the execution delay to right before the "delete" step and increase the delay as needed. For example, if you are waiting on a lot of silverlight data to build where this error is occurring, setting an execution delay for 10 seconds (10000 ms) usually allows enough time for the item to build.
To determine further what could be causing this we would need to see either the error log, the test itself so we can run it, or both. The more information we are provided with, the better, but I can understand not wanting to post your project on a public forum. If you need more help with this issue, please provide the full error log when this issue generates.