I think they had an issue with the methane sensor. From India's Mars Orbiter Mission Has a Methane Problem:
The problem has to do with how the instrument collects and processes detections of methane in the atmosphere, a technique known as spectroscopy.
"Imagine that you hold your hand in front of you and extend your four fingers ... Suppose that each (finger) represents a methane line. What they have is a spectrometer that can be shifted to ... sample each one of the four fingers and then they have a second one that samples the region between the fingers.
"The trouble is they don't actually send back the spectra. What they send back is the two numbers - the sum of the fingers measured by the first channel and the sum of gaps measured by the second channel - and then they take a difference of those two numbers and they think that that's going to be the methane signal," Mumma said.
"The problem, of course, is that when you have other spectral lines ... like carbon dioxide lines which vary widely with temperature in terms of their intensity, then those two numbers ... don't represent methane alone. The net effect is that there is no way that one can back out those two signals in order to retrieve a methane signal," he said.