When I try to add fillers in my design, there are some gaps between standard cells that can not add fillers into.
What may be the problems?
If you routed before placing fill cells, there may be routing in the gaps that prevent the fillers from being placed.
The normal flow is to perform timing optimization, place fillers and then route. If there are timing violations post-route, you remove the fillers, perform timing-opt again, place fillers again, and then do an eco-route. If you use the command setFillerMode to define the fill cells and some other filler behavior, the removal and replacement of fillers is done automatically when running optDesign -postRoute.