Here is how completion in company works (I am simplifying a bit; a more detailed description can be found in documentation of company-backends C-hvcompany-backendsRET.
You have a list of backends (completion engines) which company should try to provide completion at point. Company tries each of these backends one after the another, and the first one that returns any candidates is used for completion; the rest of the backends are ignored. So by default, only one backend is used at a time.
In your case (you seem in emacs-lisp-mode), emacs-lisp backend is returning candidates for completion, so company-dabbrev is never used by company for completion; hence, you never get foobarsentence in completion. So, you might want to change company-backends so that company-dabbrev comes before company-elisp (or company-capf in recent emacsen). But I guess that is not what you want, since that would reverse the current situation and you will not get completions from emacs-lisp backend (in which case, read on).
Company provides a way to merge completions from multiple sources using what it calls grouped backends. Usually, the members of company-backends are individual backends, but it can also be a list of backends; in which case, the completion from the backends are merged provided they return same prefix or the text to be completed (please see documentation of company-backends for detailed description).
So, if you want to merge completions from company-elisp (on recent emacsen company-capf is used) and company-dabbrev, simply do this:
(add-to-list 'company-backends '(company-capf company-dabbrev))
Additionally, you can merge different backends using the :with keyword
(add-to-list 'company-backends '(company-capf :with company-dabbrev))
This is different from the previous example using a list of backends without :with, since company will use only the backends before :with for determining the prefix (the text to be completed). This implies that the candidates from backends after :with will be ignored by company, irrespective of whether the backends return a prefix or no, if none of the backends before :with return a prefix.
You might want to set this locally in emacs-lisp buffer (I prefer doing so).
