[Koha] biblios search not working through OPAC

Hello, I have koha 3.00.02 installed on my system. Previously everything were working fine, then this strange problem came just a month ago. There are ~ 6000 records on database when i search for some word through opac only a certain results are shown which are added later which is after 5123 record. But other records before that are not shown and total results are ~400. When i did the same search through yaz-client it showed ~2000 hits. And there are ~2000 records searching on database directly. I tried to rebuilding zebra index ... upgraded system to newer 3.06 version [ as separate instance ] but still problem is still there. please help Diwaker