You can try to increase the amount of backbone and insert you are using. Typically 50-100 ng of the backbone is enough, but you might try to increase the amount. For such small inserts, it makes sense to increase the molar ratio of insert to backbone to 5:1 or even higher (since your fragment is quite small).

You can also double-check your DNA fragments by running an agarose gel. Make sure your bands look clean, no smear.

If you are using chemical competent cells for transformation use all of the reaction mix for transformation without further treatment, if you are using electrocompetent cells, desalt your reaction mix first and use a maximum of 5 µl. Electrocompetent cells typically show higher transformation efficiency, but are sensitive towards salts in your reaction buffers (causes a short circuit) and don't work very well with high volumes.