dummy regression - unequal sample sizes


I am conducting a linear regression analysis in SPSS with different variables:
- scale,
- 2-level nominal,
- 5-level nominal (dummy)
The sample sizes / frequencies for three of the five levels of my last dummy variables are under the recommended 15% (namely 12,5%, 5% and 5%). Can I correct for this? How should I do that? Can you bootstrap certain subsamples?

Many thanks in advance!
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