Are trailer brakes adjustable?

In August I had the wheel bearings packed and afterward, on the way to storage, I noticed that the brakes were not nearly as effective as they before. Even with the controller turned all the way up they weren't right. My first thoughts are either grease on the shoes or they are out of adjustment.

The guy at the shop that did the packing just told me that trailer brakes are not adjustable. Is that right? The trailer is 75 miles away and I will not use it until January and then I will take it to a shop to investigate this problem. I want to be knowledgeable when the time comes.

Thanks.