It's hard to tell what's going wrong without a MWE, but the narrow left margin might be due to the fact that your biblatex style doesn't account for beamer bibliography labels. This is the case for any standard style other than the numeric and alphabetic variants.
To fix this problem and increase the right margin, you can redefine the bibliography environment in your preamble.
With styles that already print labels in the bibliography, the
text beamer template is preferable:
Under any document class, biblatex increases the left margin to accommodate the widest label among all entries represented in the
bbl file. This rule holds regardless of whether or not the entry with the widest label is printed in the bibliography.
Other beamer templates make any numeric or alphabetic citation labels rather meaningless, unless you edit the style to print the labels with the bibliography items. In any case the label width needs to be adjusted. This can be done by adding the following patch to your preamble.