Something might be desolving in your samples. Did you seal your samples or leave them open to air?

I imagine it would depend on the acid and the base, some acids, especially the halogen acids are volatile and will evaporate, whilst sulfuric, selenic and telluric acid are very high boiling and greatly less volatile.

Some bases, the alkali hydroxides in particular are very hygroscopic and will absorb atmospheric water, coult this perhaps be the cause?

as specified already, you have to tell us what kind of acid or base you have
that makes a big difference, but seems like mad scientist got your answer already