Sometimes longer texts coming from an Excel connection, are truncated in length. Not all text is visible in PowerPoint. This happens when the Excel drivers makes an incorrect conclusion by scanning the first 8 data rows to determine its data type.
When the first 8 rows is not holding text longer than 255 characters, then internally the driver will use short text fields (max 255 characters) and cut off possible later and larger values.
You can influence the number of rows that Excel is scanning to determine the type of data. By default, this number is 8. You can higher this number (up to 16) or set it to 0 for a full scan. Microsoft warns that putting this value to 0, could result in performance issues. But we have have never seen big performance issues with this change, so go ahead, but keep in this mind. To change the number of rows to scan, use the registry editor.