I suggest you download an amsler grid to determine if you have visual distortion. You might also ask for a visual field test.
re: "neither the surgeon or the optometrist seem to think there is a problem"
With some visual artifacts after surgery, doctors label the issue a "dysphotopsia" due to the optics of the lens that you might adapt to and don't consider it "problem" at first since with many people it goes away with time (due to healing and neuroadaptation). However in this case the disappearance of part of a letter isn't like anything I've seen mentioned as a dysphotopsia they might expect you to just adapt to, though I suppose its possible they think something say in one eye being blocked will eventually be tuned out and you'll just see that bit of the world only through your other eye (like the blind spots everyone has in each eye which we aren't aware of). I'd suggest having them explain clearly what they think the cause of it is and why they expect you to just adapt to it. (phone or email might get an answer faster, presumably they don't need an exam to tell you what they already found)
re: "could it be something else now"
Obviously It sounds like there maybe something blocking part of your vision. I don't know if a wrinkle of the capsule could cause it (I'm just another cataract patient who has done research, I hadn't looked into that) , or some issue with the YAG, which would lead to that. It is possible some eye health issue arose unrelated to the surgery or YAG, since coincidences do happen.
As the prior poster suggests, it sounds like a glitch in some part of your visual field, so the question is what part of it is being blocked. That is assuming you mean it is like something is blocking part of the letter, as opposed to merely a small letter being blurry and perhaps your brain trying to interpret the blur leaves off part of it? Is it always in the same place in your visual field, the same angle, or is it say all letters on that line on the chart, i.e. the same small size? Can you see this visual glitch yourself trying to look at a book? It seems like more tests at a doctor are required to determine what it is, but more specifics might help if you wanted to see if anyone on this site had encountered the issue before.
If our current surgeon and optometrist can't give you a satisfactory answer (and you can post again to see what people think if they try to explain it, and in case someone else ever has that issue), I would suggest trying to find another surgeon to get an opinion. I don't know what city might be near with a good doctor, but some cities have magazines that publish 'best doctor' lists as rated by other doctors (perhaps search for "best doctor" and your city name and "cataract surgery"), or you could search the cataract surgery trade literature, like CRS Today:
for the name of your city to see if there is a doctor that others in the industry thought worthy of asking for their opinion for.
I have experienced dysphotopsia, and it don't think that your symptoms describe this condition. Self evaluation with a free download of an amsler grid is simple to do and can be easily repeated over a day or several days. Remember, your surgeon is looking IN and your are looking OUT. His view is objective -- he sees a centered and clear IOL. Your view is subjective -- you see blurry areas. The amsler grid sets a common standard with which you can communicate your perceptions to your doctor. Good luck and let me know about your progress.
If you are having part of the letter you are looking at missing, then you should get an OCT of the retina by an ophthalmologist or a retinal guy.
To brush off missing part of the letter is really not acceptable.