When Analysis makes assignment suggestions it looks for possible assignments within certain tolerance limits. By default these are set quite low, at values which are suitable for solution NMR. For solid-state MAS spectra it is often useful to use rather larger tolerances. You can set these tolerances by going to the Experiment menu and selecting Spectra. Then go to the Tolerances tab. The top part of the pop-up shows the Assignment Tolerances for the selected spectrum and you can edit them. Below you can see an overview of the tolerances in every spectrum. On the whole you will want to increase all tolerances in all spectra. The easiest way to do this is to set the tolerances for the first spectrum at the top (I usually use 0.5 ppm for all 13C and 15N dimensions). In the lower panel you can then select all spectra (make sure that you select the first sepctrum last, so that it is a darker shade of pink) and then click Propagate Assignment Tolerances. The tolerances from the first spectrum will now to copied to all the other spectra. Note that you can temporarily double the tolerances which you have set by clicking on Double Tolerances in the Assignment Panel.