There are different reasons why a custom field is not available for mass update. Below are some of them.
I. Custom fields must be displayed on the preferred form in order for the field to be available on the mass update. Fields on the Mass Update Fields tab depend on the fields set to show on the preferred form of the record. To resolve this, perform the following steps:
1. Navigate to the list of forms. Eg:
Customization > Forms > Transactions Forms
Customization > Forms > Entry Forms
Customization > Lists, Records, & Fields > Record Types > Edit custom record > Forms tab
2. Edit the Preferred form for the record (e.g. Opportunity Form).
3. Click the Screen Fields tab.
4. Locate the subtab where the custom field is set to Display (e.g. Main).
5. Check the Show checkbox for the custom field.
6. Click Save.
II. The custom field has no sourcing relationships. To check this, do the following.
1. Navigate to Customization > Lists, Records, & Fields > Entity Fields.
2. Click the Type column header to sort the list.
3. Look for Type = Free-Form Text and List/ Record because these fields are most commonly used as dependent fields.
4. View all fields which you suspect to be dependent to your Lead Source.
5. If the Custom field Type = Free-Form Text, go to the Validation and Defaulting tab and check if Formula = True/ checked.
If the Default Value involves Lead Source, then it is a dependent field.
6. If the Custom field Type = List/Record, navigate to Sourcing & Filtering tab > look at either Source List and Source From.
If Lead Source is the value for sourcing, then the custom field you are viewing is dependent.
7. Check the other suspected field and perform Steps #5 or 6.
8. If checking of possible dependent fields is done, make a decision or choose one of the following:
a. Remove dependencies/ links of the custom fields, so that Lead Source field will be available for mass update (**Important note: Consult your Administrator/ management if you are allowed to edit the custom fields because all records using it will be affected, and its existing values will be deleted).
b. Keep the Custom fields but the Lead Source field will not be available for mass update
9. If users choose 8.a, edit the dependent Custom fields and remove the values in the Validation and Defaulting tab, and Sourcing and Filtering tab.
When finished removing all the dependent fields, Lead Source will be available in Lists > Mass Update > Mass Updates > Mass Update Fields tab.
10. If users choose 8.b, they cannot perform mass update. Work around is to update the Lead Source field via CSV import.