They might learn from other birds, but the clear plexiglass confuses them. Try adding a few small strips of colored tape, like electrical tape, to give them the impression of a surface to look around. That has worked for some people. In warm weather, the mealworms are more active, and when they see them moving, it motivates them more to figure it out. Once the first bluebird learns how to get in, the others will learn too.
Once they learn, gradually remove the tape a little bit at a time, so it doesn't get gummy. You don't want to leave that on long-term.
BlueBars

Ok, bluebars. I tried putting tape on the feeder. First feeding in the am, I left off the one side of plexi. Its -7 here this am so I don't want to make it too hard for them first thing. It seems with the one sode off, all blues except one female go in there... I'm not sure she knows the worms are edible ... she prefers sunflower chips for some reason. So, this afternoon when I get back from errands and when its warmer (15 or so), I will go out and put the other plexi side on and see if they go in. I saw a couple attempt to go in the end holes, but only the carolina wren was brave enough. I'll keep you updated, thanks for the tip. Jenn in NH

After having the plexi on for a half hour at about 1:00 pm, only one bluebird was able to get through the hole... all these blues (9) are from broods that were hatched here this summer, I presume, so they are all around 6-7 months old. They don't seem to be able to fit through the 1.5" plexi hole. Maybe they are too fat? LOL. I could make the holes a little larger for them to fit through... what do you think? Its like they can't get their wings to fit. I don't want them to injure themselves trying to get food. I am sure its just that they are not used to going through a hole like this... to get food. The one who made it went in through a side hole (where the tape is) and then out through the end - solid. What to do?

Just an idea...Could you replace the plexi-glass with wooden slats? I made mine from instructions on Fawzi Emad's website
bluebird feeder instructions and have had no trouble with them getting in or out of the slats or the 1 1/2" holes. Unless you have special equipment, I would think it would be impossible to enlarge the holes with a standard hole saw after there is already a hole there.

I could put wooden slats in there, but when I open the top of the feeder, one whole side of them would fall out. For now, I have one whole side out and it seems to be keeping the european starlings at bay. Every time I turn around, there seems to be more and more of them; they keep the blues away and the flocks of siskins are scared of them, too. The carolina wren is a toughie... he will hop right over to the feeder, and pay the starlings no mind!
My husband has every tool known to man, power and otherwise. I would think that he could make the holes bigger for me... by maybe 1/16 or so. Thanks for the help!

UPDATE!
I had my husband enlarge the holes on the removeable side of the plexi a bit (with a dremel type tool) and the blues have made 4 entrances and exits so far. It seems to be working.

Glad its working, and hope the darn starlings don't find it!
Bluebars

You know... I never counted on starlings having such long necks. Evidently, even without making the holes larger, they can reach the mealworms, so this feeder is a $30. paperweight. Besides the fact that every time the blues swoop down to eat anything, the starlings also fly over to the feeder and scare them all away. The blues take too long to enter the feeder and give starlings time to fly over from a nearby bush. Maybe the Bluebird Nut feeder is the way to go. This is driving ME nuts since I have never had a starling problem.

Jenn...I don't have starlings, but I do have a great mealworm feeder for the blues(though I've not had any mealworms in quite awhile !) It's the Bluebird Nut Deluxe feeder, a round jailhouse style with the cup in the middle and metal top. It can be hung or mounted. I think someone on the forum recommended it.Â

Info about the feeder says it's starling proof, and it will allow smaller birds to enter, but not the larger backyard bullies! I know you'd rather alter your existing feeder than purchase a new one, but just wanted to mention this feeder to you in case you were interested. Good luck!Donna

Yeah, Donna. I got the bluebird nut feeder... but just like the other feeder I had to enlarge the holes on, the blues are acting like they are too fat to get in there. They mill around and hang on the cage... I did squeeze the bars apart on one whiole row so they are larger, just like the instructions say to do for mountain bluebirds... I hope it just takes time for them to figure it out.