If the NextGen system came online recently, what they did was make your TRX scanners not fully compatible with the system they are now using. The NextGen system is simulcast. Whistler was working on a new generation of scanners to be compatible with simulcast systems like NextGen (called the TRX-100/200), but they couldn't overcome the technical challenges and cancelled the project. As others have stated above, the Uniden SDS-100/200 and Unication pagers are built to be compatible with simulcast systems.
You can try some tweaks to attempt better reception, such as attenuation or using a directional attenna, but keep in mind that your success may be limited.