Usually an undersized fan, a suction point that is not reaching the whole slab, or unsealed entry paths the system cannot overcome. A correctly installed system should be diagnosed and corrected under warranty, not re-quoted.
More detail
A post-mitigation test that still reads above 4.0 pCi/L means the system is not doing what it was designed to do, and it is a diagnostic problem with a short list of causes. The most common is sub-slab communication. A single suction point can only depressurize the area the gravel or soil beneath the slab lets it reach. Slabs poured in sections, interior footings that divide the sub-slab space, and poor or absent aggregate all block that spread. The fix is usually a second suction point rather than a bigger fan. The second is fan sizing. Fans are selected against the resistance of the specific house. A fan chosen for a permeable gravel bed will stall against tight clay, and a fan that is too large can pull conditioned air through the slab without improving the reading. The third is unsealed openings. Open sump pits, floor drains connected to the sub-slab, expansion joints, and utility penetrations let the fan pull basement air instead of soil gas. Sealing is part of the install specification, not an upgrade. The fourth is the test itself. A retest run without closed-house conditions, or placed near an exterior door, or run during a storm front, can produce a high reading on a working system. Confirm the protocol before condemning the install. On responsibility: a system installed to ANSI/AARST CCAH-2020-0523 comes with a designed performance target, and reputable installs include post-mitigation verification. Diagnosis and correction of a system that missed its target belongs to the installer under warranty. Get the guaranteed level in writing before the original work, and this conversation is short. Sub-slab communication is the usual culprit in this market specifically. Central Ohio sits on flat-lying glacial till with dense clay subsoils that drain slowly and perch water above the till rather than letting it through. Tight clay beneath a slab spreads suction poorly, which is why a second suction point resolves more failed retests here than a larger fan does.