If you've had trouble getting a stand of sweetclover established on your farm, you're not alone with your problem. The culprit is the tiny sweetclover weevil. It can be controlled by late seeding and by using DDT.
Lilly, J. H. and Walstrom, R. J.
"Case of the "Disappearing Sweetclover","
Iowa Farm Science: Vol. 6
, Article 7.
Available at: https://lib.dr.iastate.edu/farmscience/vol6/iss10/7