These symptoms sound like the disease silver leaf, which affects plants in the same family as pyracantha. This is a fungal infection, which causes the upper leaf surface to separate from the rest of the leaf, making the leaf look silver.
However, adverse weather, drought, insects and poor feeding can also cause these symptoms.
To investigate the cause, cut an affected stem where it is wider than 2.5cm, and look for a brown stain inside.
The best way to avoid silver leaf infection is by only pruning susceptible species in the summer. If you find the brown stains, cut back to 15cm lower than the stain. Keep disinfecting your pruning tools with a household disinfectant, as you prune. Burn the pieces you cut away.
In the meantime, make sure your shrub is well-fed and well-watered to give it the best chance of recovery if it does have silver leaf, or to avoid these symptoms if they are simply caused by plant stress.