[mention]Tapion[/mention]
Dabura was probably holding back, as it is established that Majin Boo's revival would require an extended, drawn-out altercation. Defeating Gohan quickly would result in no energy for Boo.
But why would he hold back to a level where he didn't even damage Gohan? What was established was that the only way Boo would receive energy was if Babidi's warriors managed to damage their foes. Rusty or not, Gohan was still exhausted enough to warrant taking a Senzu bean after the fight, implying that the fight was fairly long and tested him. If Dabra was suppressed, it should have been easy for him to up his intensity a little bit to actually deal damage to Gohan.
Later, we see Kaioshin saying that Gohan would need to get angry and release his true power to have a chance of defeating Dabura:
Kaioshin: “Goku and Vegeta are probably having a roughly even fight…I’ll pin Babidi down. After that, if you can manage to defeat Dabra, then Babidi’s scheme will be completely wiped out! So then! Now’s the time to get angry, like Goku said! Get angry and show me your true power! Gohan, peace is almost at hand!”
I'm not sure this contradicts my stance. Gohan was shown unable to defeat Dabra earlier despite not taking any serious damage, so it stands to reason that he would need his true power/anger to defeat him.
When Boo first appears, Gohan is blown away by his power and states he would need to put out the exact same power he used to kill Cell in order to defeat Boo:
Gohan: “…Is that so?...He really does have tremendous ki, but…It doesn’t seem like so much so that there’s nothing I can do…[ ] …If I can only put out all of my true power…”
Dabura, on the other hand, explicitly comments on Boo's power and calls it garbage:
Dabra: “I don’t know why, but it seems we failed…I thought [Boo] had finally appeared, but it’s just some stupid-looking raw garbage, without brains or power…”
Boo has to perform a massive power-up before attacking Dabura, one so great that it shocks everyone in the area:
Gohan: “It-it rose…Majin Boo’s ki rose explosively…He’s str-strong…Too strong…This is unbelievable…”
I'm not so sure of Dabra's ki sensing abilities or how they work. The first glimpse we get is Dabra being able to sense all 7 fighters outside of Babidi's ship, including Kibito and Kaioshin who have insensible ki. We also know the Z Senshi were suppressing their ki to 0, yet Dabra was still able to discern those with "great potential". So how is he able to gather this when their ki was suppressed?
So already we can establish that his sensing abilities work differently. Next, we have Goku transforming into Super Saiyan against Yakon.
It's pretty odd that Dabra's first assessment of SSJ Goku was that he was glowing. He had no idea that Goku powered up until Babidi looked at the kiri meter. We later see the same type of reaction when Boo was forming:
Gohan immediately was able to tell that he hadn't destroyed Boo at all. Both Dabra and Babidi had to use Gohan/Kaioshin's reaction to validate Boo's existence. They seemingly couldn't sense his power forming right above them. Dabra once again failed to comment on the size of the ki, but commented on the smoke forming. It makes me really doubt his ability to sense, and makes me think he was judging Boo initially by his appearance and retarded personality.
Unfortunately we only see Gohan react to Boo's power up, so it's unclear if Dabra was aware of it before Boo one shotted him. Either way, I think this evidence throws some serious doubt into his abilities to sense ki.
