I dont know, never been to your dojo - so its a hit or miss.
Could be 'relevance'.
From my understanding people come out of interest and then give up on Aikido not long after.
They either think 'it does not work', or they last long enough to see its value, but think it would take forever to apply it in a real situation.
A lot may have to do with communication.
Pep talks aside, address the issues mentioned above.
Ask people for their honest feedback and watch where they are at.
Do they look like Aikido looks like a joke to them, etc? What is it that is causing their interest to go down?
Perhaps they learned 4 of the more common Nage moves and same with katame waza and they think this is boring now.
I see this...
Taking aikido and breaking it down into sections and stages of obtainability - focusing on what you can expect once you achieve the goal, of say black belt, would help.
(This will vary by mileage, but the concept is there.)
I would say also in this day and age, Aikido may be at even more of a disadvantage of front as to whether or not it will keep ones interest on the perception that it is fake...or useless.
If beginners realize that not all attacks look like a 'fight club' and that it starts with pushing, shoving, or yelling sometimes - that Aikido blends well in certain circumstances.
And past those circumstances, what is Aikido drastically changes in form, but the concept is their behind it, when in a fist fight, per say.
Once you apply the concepts and you get them to the ground...you have it made. Now some may well argue that aikido is not a ground sport - but I would say the key is to look at it holistically.
In this day and age, this may be what the issue is.
Take a look at what was said, see if it makes sense at all, and this may or may not help to some extent. (Most of it applies to the teacher as its up to them whether or not people come. They are the leader per say.)
But gone are the days of just taking a martial art class cause you saw 'karate kid' and making you look tough. (suppose Im talking about the younger ones that end up in the dojos and leaving.)
And for grown adults, the value in Aikido often is not conveyed.
I had to figure it out for myself...many dont take the time to see what they can get out or make out of something...so if you, or your teacher wants to keep students, thats something to think about.
And last but not least...enjoy your time with the teacher than...learn what you can from him. But I suppose if its a full class you will not get one on one, and you are missing a more senior mentor while practicing. Perhaps he should give you some one on one if this is the case where your basically the only one that shows up and continually goes...(past the new guys that come and go.)
Best to you - most of all, enjoy what your doing in the 'now'...make the best of it...its all you will ever have.