Negatives generally do not become noticeably blurrier with age. Poor storage can cause fading, color shifts, or physical damage, but loss of sharpness is usually minimal unless the film was stored in very bad heat or otherwise badly damaged.

If some scans from the same batch are sharp and others are soft, the most likely cause is the scan, not the negative aging. A common issue is scanner focus: if the film isn’t held flat or the scanner is focused incorrectly, a higher-resolution scan can still look blurrier than an older scan from a print.

Also, print scans can look “sharper” because the original print-making process may have added contrast or other corrections that improve apparent sharpness. Lab printers often optimized prints automatically, so a print scan may sometimes appear crisper even if it contains less true detail.

Disc film can indeed be limited by its very small negative size, but that would not explain soft 35mm scans if other 35mm frames scanned well.

So: no, negatives normally don’t lose resolution over time in a way that explains this. The evidence points more toward inconsistent scanning quality or focus, and it would be reasonable to ask the lab to review or rescan the soft frames.