Vigorous annual growth among flowering cherries varies, depending on certain factors relating to their location. Is the tree in a spot that is too shady, for example on the north side of a building? Flowering cherries love the sun, and they will definitely perform better if they get enough of it. In which case, would it be possible for you to relocate the flowering cherry to a different side of the garden?
Is it in poor, sandy ground? If it is, it may be hungry or drying out in the summer. One remedy is to pierce the ground around the tree nice and deep with a spade, about 50 cm away from the trunk, in late August. This will help new roots to form. Then, in early March, improve the soil by digging in plenty of organic material (soil:humus 2:1) and a handful of lime to a good depth to invigorate the plant. Also make sure that you add fertiliser regularly – either one to two litres of compost per square metre, or 50 grams of compound fertiliser per square metre. Apply an annual light feed of lime (50 g per square metre). Don't forget to water every now and then – especially on poor sandy soil.
Or it may be that your flowering cherry is in compacted soil. If it is, loosen up the ground around the plant as deep as you can manage using a bar spade. If that doesn't do the job, try using the "renewal process" mentioned above, making sure that, in addition to improving the soil (in that case, add a barrow load of course sand), the compacted soil is lifted really well by digging deep to break through the compacted ground layers so that the roots of the flowering cherry can penetrate the soil properly.
And just one more thing: If when you bought your flowering cherry it didn't have bare roots or wasn't in a container, but came instead as a root ball, check whether the linen of the ball had been knotted around the stem of the plant. If it had, then over time the trunk can become constricted, stunting the plant's growth.