It is difficult to say for sure but if you are not letting it dry out then perhaps overwatering is the cause. If the container doesn't drain properly it may be that roots are sitting is soggy conditions which is deadly to them. If the plant is able to recover you should consider moving it to a little more daylight unless it is a species that doesn't want too much light.
It is also possible that the dry leaves are from some past drought or drowning conditions and now that you are providing adequate water it will resume new growth if the damage was not too severe.
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