The answer is 'Channel overlap from neighboring access points' because in densely populated office environments, multiple wireless networks might be operating close to each other. If multiple access points are set to use the same or closely overlapping wireless channels, their signals can interfere with each other, leading to poor connectivity and performance for users. 'Construction materials causing signal reflection' might cause issues but is less likely to be the new source of an issue if the wireless network was functioning properly before. 'Increase in the number of wireless clients' could cause congestion but would not typically create interference and the question states the new devices belong to a different company and are likely not using the same network. 'Firmware updates on the access point' could introduce new issues but would not typically cause interference which is usually related to competing wireless signals.