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If a user types in a keyword which is exactly the brand name, then he must click on the result as it will definitely be on page 1 at position 1
The brand-name will of course affect this, as a business name made up of other search terms (e.g. mytown fast clutch repairs) couldn't expect the same CTR as something unique that does not involve common words (e.g. Imzinatia).
However, even with something like the latter example some searchers may be looking for independent reviews of the business rather than the business itself (or might go to the Facebook page - if there is one - rather than the main site). You couldn't therefore expect 100% CTR even in the case of a reputable business with a unique name.
Posible reasons for lower CTR might be:
1. The brand-name is similar to that of a very popular business;
2. Your site has poor reviews somewhere, and users are looking for corroboration;
3. Users are opting for one of your competitors in Google's "people also search for..." box;
4. A review site is in a prominent position (e.g. #2), and users want to look at that first.
You don't say what is in the following postions on page 1, which will obviously also have some bearing on it: in Google searches for my own (unique, registered trade-mark) brand-name is #1, but is also highly prominent (six popular sub pages are listed in a block immediately below it), while positions immediatley below it comprise directory listings, Facebook, and other relevant pages. I think it unlikely that a click on one of the popular sub-pages would be counted in Google's CTR (and Facebook obviously wouldn't), and if your site has similar pages in Google's listing for your barnd-name that would have an obviouls diluting effect.
There is also the point that Google's stats are notoriously unreliable, and have been bug-affected recently, so I wouldn't be unduly concerned myself unless I saw a significant downturn in traffic.
However, if the CTR is very low and it isn't just a reporting issue you may need to do some work find the reason for it.