Maybe some more details will help identify the problem. For example, what is the application, why is a fuse used when the VCB already should have short-circuit breaking capacity, what is the Fuse rating and VCB rating, is a switching-on surge coming on the system.. so on.

If only one phase Fuse is getting failed, there may be chance of IR value of that pahse cable or the feeder.

If fuse failing on random, it may be happening due to inrush currents, to minimise the inrush currents, you should operate the distribution transformers without load during start up and during changeovers. and it would help the distribution transformers operated sequentially