Fiber Trouble

I just connected 2 of my company’s buildings via fiber. All works well except when we try to transfer large databases from one building to another, the connection then times out. I also notice that when I ping with packets of 2000b or larger, about 10 go through, but then every once in a while, they time out. I changed the fiber module jumper from Full to Half duplex, this seemed to help a little but we’re still having trouble. This is single mode fiber and the buildings are close. Can anyone please help, do I need a fiber attenuator? Thanks.