Load-balancing

Currently we are having 8xE1s running to 2 different ISPs. 4xE1s to each of them. The frist 4 links were established about 2 years ago and connected to 1 ISP. Due to congestion, we add 4 more links but to different ISP. All 8 links are running on BGP. The load balancing on the first 4 links are working fine but the load-balance on the additional 4 links are not working. At the same time, the traffics are not load-balancing to both ISPs. The first 4 links are still 98 percent utilised while the additional 4 links are only 20 percent utilised. I’ve tried prepending the AS numbers but it doesnt seems to work.

Can anyone help me in this issue.

Thank you