A few years ago my right shoulder had bursitis and I could barely move it, if at all, due to excruciating pain. I know what you are mentioning is probably more subtle compared to that, but even today my shoulder isn't back to 100%. However, when my shoulder gets stiff or feels like its weaker, I usually do 5 different exercises and within no time my shoulder feels back to about 99%. The exercises I do that should help you as well are:
- Front Dumbbell Raise
- Dumbbell Shoulder Press
- Dumbbell Side Lateral Raises
- Bent-Over Dumbbell Rear Deltoid Raise
- Dumbbell Upright Row
I usually do three to four sets of these each, from 10-20 reps depending on the weight. Definitely start with light weights at the start until you feel comfortable going up.