Several breeds of dogs including bulldogs are prone to alopecia (hair loss) on their sides or flanks.
When this is seen the dog should be tested for hypothyroidism (low thyroid). If the thyroid level is normal, then the hair loss is probably seasonal alopecia. There will be hair loss in the time of the year with shorter days and initially the hair will regrow in the time of year with longer sunlight days.
Seasonal alopecia is usually progressive and eventually the alopecia is all of the year.
The disease progression can be slowed somewhat by diagnosing and treating all other skin conditions, omega fatty acid supplement and not over bathing.
There is an expensive treatment for seasonal alopecia that almost never works. It is best to just accept the bare flanks.