Maybe try your LPS on another cell line, like macrophages where you would forsure get a response, if this works then it is not the lps. Have you also controlled the viability of your bv2 cells? have you changed anything in the conditioning medium? If you got high inflammation only once and then never again, it might mean that during that one time something else might have caused that inflammation, like not using something sterile for instance.

If your LPS had worked for you before for some experiments, then it suggests either the cell line or the LPS have undergone some changes, sufficient to give you a report that is different.

Make sure that your LPS was initially prepared at a high concentration as a stock solution for storage. Typically, it should be 1-5 mg/mL or higher in endotoxin-free distilled water (sterile). And, the stock solution should be stored in small aliquots to avoid repeated cycles of freeze-thaw when you use one of them.

The stock solution should be stored at -80C if possible. When you take out an aliquot (typically, I would say 20-30 uL per tube is enough), and when you are done with pharmacological challenges, the unused part should be stored unfrozen at 4C, until the next use. (And it should be finished off being used soon).

LPS prepared at below 1 mg/mL and stored unfrozen is known to be absorbed and therefore lost to plastic surfaces of containers (tubes, etc.). Your effective doses would no longer be predictable.

In short, freeze-thawing and liquid storage are both non-ideal for long term use of LPS.

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Separately, with the cell line, check your passage number. Over-passaging alters the sensitivity of cells to immunostimulants (usually lowering it). Even with storage in liquid nitrogen, over time, viability and quality of the cell stocks degrade, you may have to check and consider re-making a good new batch of cell stocks from time to time (typically 2-3 years after the initial batch). A bad accident of liquid nitrogen depletion for one night, is enough to adversely affect many cell stocks' quality.

As a last resort, write to other labs to request new cell stocks or purchase some if commercially available.

Good luck