I cannot fathom why Theon would not ask Daenerys for help to save his sister in Game of Thrones season 7. After all she was her ally. She would have every reason to help. But Theon instead went to smack another man's face and take over as the leading character of their group, so that he could risk his and their lives by attempting to save Yara with such mediocre manpower. Why?
Note that Theon was concerned about Yara's subjects (the Ironborn) not being all that interested in rescuing her (from what we see). Regardless of Dany's help, Theon needed to unite the Ironborn. Because if he doesn't, then they might not follow Yara if she comes back into the picture.
When you say "mediocre manpower", does that mean you think Theon only has the support of the 15 men that saw him beat up that guy? This group symbolizes all the Ironborn (at least those that followed Yara), Theon has essentially proven himself worthy of leading all of them in Yara's absence. Similarly, we often see Grey Worm speak for all of the Unsullied (he represents them), and Thormund speaks for all of the free folk as their representative. This group of Ironborn that witnessed Theon's fight similarly represents all of Yara's troops.
The first part to this answer is that Daenerys going after Theon in order to save Yara could and would be taken as an act of hostility towards Cersei, and would have ended the armistice between the two. Basically, the Mother of Dragon's hands are tied behind her back from helping her allies.
The second part to this answer is, maybe, this time, Theon will finally grow as a character and stop being a coward and do something besides kill defenseless children. Yara, even after all of Theon's mistakes, was the only one that was willing to forgive him and let him atone for what he did.