i would like to see a pic of the whole...

to try to determine.. if they are the outer.. oldest leaves ... and at the same time.. to find out.. how the active new growth is reacting ....

and if they were just older leaves ... i would just prune them out...

no plant holds its leaves forever ... and it is just annual maintenance to remove such ... and i cant tell from this pic ...

also.. any history of moving it.. like bringing it inside for winter .. or some such.. that would trigger thoughts of severe changes in light.. and humidity ... etc ... hardening off issues ...

and finally ... it i saw this.. those other things ruled out.. i would look to water management... either too wet.. or too dry.. for a short period ... and the plants reaction to such ....and again.. it would be the older leaves ....

and then.. finally.. if we fail you.. you might get more specific information in the houseplant forum ....

if there was a stress... that causes a problem.. we need to define the stress ... the CAUSE ... rather than dealing with the EFFECT ... which may or may not be a disease.. because unless the CAUSE is addressed... its useless to cure the EFFECT .. if that make any logical sense ...

soooo .... we need more info ...

ken

Thank you for replying Ken. It was standing next to another Calathea that was suffering from red spider mite and I think it might have caught some. I have sprayed the leaves with a mixture of water, natural soap and alcohol.
I have checked the bottom of the leaves and it appears to have helped. But ever since these brown spots have been developing. I am keeping the soil relatively moist, watering every 1-2 weeks and preventing it from drying out.

I hope this gives a clearer picture of what might be wrong and I hope you can help

If it's taking up to 2 weeks for the soil to dry out, the roots probably don't have enough oxygen to function. Are you able to slide the root ball out of the pot without the dirt falling apart?

thats a darn good looking plant.. cut out the 3 worst leaves ..

and then look at the media as purp notes ...

ALL evergreen plants.. shed old leaves ... and it starts by looking like yours does... the leaves just shut down.. start browning.. etc ...

and if that is the case... you have used a lot of cures ... for the mites ...

i just dont see that big an issue...

try not to kill things.. with too much love ...

good luck

ken