When a jobs fails and is restarted through Modify > task, it takes several minutes to actually activate and show up in the activity window.
There are errors on the database side that is causing the slow activation, particularly on the AH table.
A common message that should be seen in the logs is this:
20180426/221247.795 - U00003434 Server routine 'JPAKTJ_R/AKTX' required '4' minutes and '9,614' seconds for processing.
As this server routine specifically has to do with job activation.
-Ensure maintenance is done regularly
-Ensure that fragmentation is low on the AH table (we recommend under 30%)
-Make sure there are no Oracle restore points on the database. If there are, then delete them - this can cause extra stress on the database.