Hello,
I guess by seeing alerts but no notifications, you mean the trap is identified by iMC in Trap Browse, but does not generate an alarm. That would make sense, as iMC has a definition for the ErrantBDPU Traps (Trap OID 1.3.6.1.4.1.11.2.14.11.5.1.7.1.0.6.1), but by default does not trigger an alarm for them.
To do so, simply create a new Trap to Alarm rule that matches by Trap Type on the Trap OID 1.3.6.1.4.1.11.2.14.11.5.1.7.1.0.6.1:

It will be Enabled automatically, and trigger an alarm when such a trap is received. Note that the level of the alarm generated is determined by the severity of the trap - for ErrantBPDU that is Warning by default, and could be changed via Trap Definition page.
Also note that "filtering" eg. Filtering Trap page mean that you 'filter out' certain traps, so you don't see them nor get alarms from them. If you have defined a filter for the ErrantBPDU traps, then you should remove it, or you definitely won't get alarms for them.
Hope that helps.