I agree that your sample maybe contaminated with bacteria.

Have you tried new membrane?

There may be incomplete blocking of the membrane. You can use non-fat dry milk (Sigma Aldrich) for blocking to prevent non-specific binding onto the membrane. Use fresh membrane it may be contaminated and use freshly prepared solutions like blocking, washing, and an antibody solution. Please also check and optimize these important points:

1) Increase blocking time

2) Reduce primary and secondary antibody concentration

3) Washing should be proper

4) Reduce exposure time.

5) Never handle the membrane with barehanded, always use forceps.

I hope this will be helpful.

Goodluck.