97 S-10 shifting problems

My 1997 S-10 started having clutch problems about 9 months ago. Since then it is a routine that I fill the Hydraulic Clutch Reservoir before every start.So long as that reservoir is full my shifting is smooth sailing. Recently that all changed and even though the reservoir is full my truck is not leaving first gear, it recognizes no neutral. I have not as of yet tried reverse but I have a bad feeling . Any suggestions?