Martin, which drug was prescribed, when was it prescribed, what dose was prescribed and how much does your dog weigh? My initial thought is that you're see an adverse drug effect.
1/4 of what milligram tablet, please? I might initiate dosing at 2.5 mg/kg or 6.5 mg twice daily for her. 1/4 of a 30 mg tablet would be 7.5 mg which would be appropriate. We do see ataxia ("drunken sailor") during the first week of phenobarbital administration but then our dogs accommodate and normalize. Yes, seizures first appearing before the age of 5 years are usually due to idiopathic (unknown cause) epilepsy. Brain tumor is the most common cause of seizures first appearing after 5 years of age.
Be patient with her for a few more days. I have to leave my computer for the night but I promise to reply in the morning if need be.