Hi, usually poor elecrical connections. Oddly the last bike I had that did this would work when cold but dropped the front-right when warm, I'm guessing the rising ambient heat caused something to expand slightly and lose decent electrical contact.
If only one not working (and the other on the same side going double-time) regardless whether you have original or aftermarket indicators, first check bulb is seated securely - sometimes a new one can help. Then check the cleanliness and security of the electrical connections from indicator unit to wiring loom. If they're mini indicators or similar they may have poor quality connectors crimped on.
Oh and if it's both front and rear on the same side intermittently failing at same time, first try blowing some leccy contact cleaner into the indy switch cluster. If that doesn't work try splitting the switch cluster on the bar and give it a good clean out. If you don't have contact cleaner WD40 will do just don't over do it.
Finally do you have LED indys, any recent damage left a drop on the L/H side and is your bike generally original, a butchered streetfighter or somewhere inbetween?
