Rechargeable batteries are sometimes a culprit. Some batteries, the ones that have the little "bump" on the positive contact, can work better than the ones without it. Possibly a battery contact inside the battery comaprtment is a bit bent-down. Re-check the flash mode to make SURE-SURE you're not doing something silly, like firing in 1/16 power mode, etc.. I've made a zillion dumb flash mistaks over the past 35 years...chwck also the flash is IN the hot shoe,alll the way, like 100%...Do'ah! (Please ask me about this one! NOT!)
(Insufficent) Power is usually the #1 problem. Contacts being ***110% cleaned** of finger oils might be problem 1B. Making absolutely SURE the settings are properly set is something I've messed up a few (dozen) times. A few to check...flash mode (A, TTL,Repeating, B-TTL,Fractional manual), camera ISO/ASA, lens f/stop, lens diaphragm actually working PROPERLY...that is one that can be tough to spot depending on brand of camera, if the lens diaphragm conks out.
Worst-case scenarion is the flashtube is mostly leaked out of the gas, and you'd need a different flash unit.