Adding on above from Mare, During maintenance Make sure flame eyes are clean also.
Ships now should be burning Do and not HFO for boiler now that IMO2020 kicks in, chances for misfiring should be lower.
Always, always Make sure boilers are properly purged everytime you fire manually, sometimes there would be backfire, its dangerous.
Not my experience, but there are instances i heard whereby boiler tubes are badly choked that backpressure causes boiler flame to diminish.