Digi One TS W

We have a digi one ts w with SSID (shared) and 128 WEP key and in our company we have 4 access point (SSID and 128 WEP key).
The digi one always only connects to the same access point and it does not connect to the 3 others AP even if it is close to the AP.
The 4 AP has the same configuration and setup.
Could you help us to resolve this problem?

Thank you.

Marcos Coelho