There appears to be a bug here in that a BWS for δοῦλος produces zero results in the BWS for the NT, although doing a "Search this Resource" from my primary Bible (NIV) or another Bible (e.g. ESV) does produce hits.
What I did:
Method 1 - Open BWS from the Guides menu and type in g:doulos and select δοῦλος from the drop down list. Press enter and allow the guide to populate. Notice that the section named "Textual Searches" shows 0 results in 0 verses. However, clicking "servant" on the translation wheel produces numerous results, in my case it showed 93 of 126 uses.
Method 2 - Open a passage containing the word servant/δοῦλος (e.g. Matt 8:9) and run a BWS on δοῦλος from the drop down menu. Results the same.
Screen grabs:
Notice results from "Search this resource" from right-click, drop down menu on "servant" in Matt 8:9.

Notice "0 results in 0 verses" in textual searches at bottom of BWS (generated from right-click menu as described above).

Notice results generated in NIV Rev Int after clicking on "servant" in the translation wheel.

EDIT: Edited title, added Bug tag for NA27 Gram/Int.