To determine if CTR is relatively low, or relatively high is by looking at the Quality Scores. Does the client have above average, or below average Quality Scores? A QS of 6/10 is average, 5/10 or lower is below average, 7/10 and higher is above average. Does your client have below-average Quality Scores?
If the client does have lower than average Quality Scores then you need to look at the account structure to make sure that it is optimized for highly specific message matching.
If the account structure is optimal, check the account data to see if various value propositions have been tested and that the best performers are where the bulk of the ad spend is currently going.
Check to make sure that the primary ad message (the value proposition) matches the most common intent of the keyword. Poor message match between the intent implied by the search term and the value proposition in the ad text can cause poor CTR.
How about message specificity? Did you verify that the ads all contain highly specific marketing messages that perfectly match each and every keyword within the ad group? If there are too many keywords in an ad group it is impossible to write highly specific messages that match each and every keyword. Generic marketing messages are notoriously poor performers. Your ad messages need to be highly specific, are they?
What about ad clarity? Is it extremely obvious what is offered by the ad? If you ask 10 random people what the ad is promoting do all guess correctly? Lack of clarity is the number one CTR performance killer. It can sometimes be difficult to judge the clarity of the ad for yourself because you already know what it is meant to advertise. You need to focus group test the ad text to be certain that it has 100% clarity for all random viewers, otherwise re-write those ads to be more clear, while also being very specific.
Those items above are just a handful of the many possible issues to consider when focusing on CTR performance. If you can provide some answers to those questions we can take a closer look at your data to get to the root of whatever issues are challenging you and your client. Just let us know, with some specific data, and we will happily help you out.