The way most indicators are set up is that the load on them affects the flash rate (those older models used current flowing through a bi-metallic strip: the bulb current heated the strip which then broke the flow of current). Modern indicator circuits are often much more electronic, but the same setup is used to make it obvious to the driver what the problem is).
As @NickC says, it's the bulb that needs replacing.
The majority of indicator problems are down to the bulb - it is a consumable - so just replace it.
Less likely is the indicator relay (see Is it possible to repair an intermittent or flaky turn signal flasher relay? for a possible repair).