Some of the dropdowns in the Select Data may be empty when you try to select a valid value. This may be true even if the individual Lookup table is populated with values. This is most likely because the values in that table are not Enabled. There are two way to correct this.

If you want all lookup values that are in each lookup table Enabled, from the EDITOR click on Maintain Database under the Setup Group then click on Enable all Lookup Values. 

If you want to choose which lookup values are avaialble, Topen the lookup table in question using the EDITOR --> Manage Lookups and make sure the Enabled check box on the form / table is checked. You can also open the lookup table in Datasheet mode and then scroll to the right to see which records are enabled. Select the ones required for your project and close the form. NOTE: you will have to refresh by clicking the Refresh Selected Data to see this changes. 

The examples below are for Sample Purpose but there may be other lookups that need to be enabled depending on the project you are working on.

The first example show that Purpose under Samples is Null when you attempt to select a value. The others show the lookup talbe and datasheet view with the enabled check boxes.