The most common problem is setting your record format incorrectly when configuring your Z39.50 connection. The four types of record formats and the types of data available with each using the Standard Connection include:

As you can see from the table, Marc records do not support text. If you have selected Marc records as your record format you will not be able to access full text.

Another problem is selecting the wrong element set.   F will only provide the full record, select T instead to retrieve the full text.