It is well established that virus titer goes down with passaging because the population accumulates inactivating mutations with each passage. One way to ameliorate this is by lowering the MOI at which you passage. If you start with a high number of viruses all the cells are infected and the progeny is highly heterogenous  but If you start with a low virus to cell ratio you select for infectious viruses. The rational is that only infectious viruses made by the few cells that are infected in the first round would have progeny in the second round and those should have a higher chance of being infectious. In addition, you avoid having too many viruses infect one cell which is an additional source of defective virus. Another thing you may consider is to plaque purify your virus. Virus plaques can be stored for very long times and give the best titer.  

It is well established that virus titer goes down with passaging because the population accumulates inactivating mutations with each passage. One way to ameliorate this is by lowering the MOI at which you passage. If you start with a high number of viruses all the cells are infected and the progeny is highly heterogenous  but If you start with a low virus to cell ratio you select for infectious viruses. The rational is that only infectious viruses made by the few cells that are infected in the first round would have progeny in the second round and those should have a higher chance of being infectious. In addition, you avoid having too many viruses infect one cell which is an additional source of defective virus. Another thing you may consider is to plaque purify your virus. Virus plaques can be stored for very long times and give the best titer.  

Many reasons.  A permissive host cell has not been found yet - cells tried do not have the receptor for the virus (cell surface tropism) and/or the cell does not have the  intracellular machinery the virus needs to hijack from the cells (internal tropism). While there is some thinking that viruses on cell culture passage attenuate - this is not always the case;  often it is the reverse.  A virus's fitness can increase of decrease or passage in cell culture or natural host.  Think of epidemics or bioterrorism concerns. In some ways it is in the best interest of the virus to create diversity.

Rabies viurs goes quite well on BHK-21 and as a matter of fact this is one of the best cell lines to this end.

Have a look on this paper

http://www.ncbi.nlm.nih.gov/pubmed/26645040

Have a look on this paper

Many reason. Dealing with Rabies virus and Rhabdo viruses generally the problem might be the production of defective interfering particles. These are incomplete virions that will block cell receptors and therefor infectious virus. The way around this problem is to dilute your inoculum 1: 1000000 or more and at the first sign of CPE harvest and repeat. This removes the vast majority of DI particles.

Why not test the limit dilution technique for particle selection that will best grow on BHK . Also what is the strain that you use?

ERA grows well on BHK21 .

rabies virus grow well on BHK cell line. many reasons for not growing..one reason might be contamination of the cell line..the other is reason might be too much virus (0.01of virus/cell) is good.