Why on earth would they want to close that if they discovered it? Seems like the perfect conduit to feed the Germans exactly the information you want them to have.
Asked that same question myself. I've found there was a large 'clean up' of intel problems during 1942, by the Brits. Trawling through the literature theres descriptions of the recognition and correction of many bad practices and poor programs. There was a shake up in SOE leadership, there was a crackdown on bad radio security within 8th Army and other commands in the middle east. The discovery of the compromised US Army Black Code was one of many in latter 1942. In part not using the Fellers messages as a section may have been the Deception Committee did not have the necessary level of control over what went into the messages. They learned the hard way such things had to be carefully crafted and controlled in execution. The Brits had been thru their own 'amateur hour' phase in this 1939-41 and were seeing the Yanks repeat many of the same mistakes.
There may have been a serious degree of urgency in the matter. The successful deception operations took months to develop and the accurate messages had to be gradually salted with the misleading information. Abruptly turning a "good source" into fraud risked being obvious to a enemy intel analysis. The Brits welt under pressure in Africa in 1942 and keeping a dangerous source open to the Axis for months while a deception op was ramped up may not have looked like a good idea. There is also that the US Army was embarrassed and in a hurry to correct the problem and the code changed before anything could be proposed.
So, my best guess is the Deception Committee focused on crafting what looked like the best shots in 1942 and avoided pursing every fleeting opportunity. Thomas Holts 'The Deceivers' is a 800+ page description and analysis of Allied deception ops. The not entirely comprehensive it is still the most complete I've found. A must have been for understanding how the system grew, operated, and how the operations were from 1942 woven into a coordinated whole. Holt lists a large number of major and minor deception ops that are not named in the mainstream histories of WWII. Interestingly the Japanese were relatively unresponsive to Allied deception ops. Sometimes the effort worked. Most examples looked at there was no perceptible result.