iBook G3 Clamshell not recognizing new RAM modules

As SDRAM evolved, high density chips meant that fewer chips on the card could produce greater overall capacity. For SO-DIMMs, this was a breakthrough. The same was true for DIMMs, but unfortunately a lot of the older Macs don't support high-density memory chips. As an example, the iMacs from 1999-2001 could not recognize high density modules. A 256 MB DIMM needed to have 8 chips on each side of the card, for a total of 16 chips. A 256 MB card with 8 high-density chips on one side wasn't compatible. A compatible 512 MB DIMM had to have 16 chips on each side, with 32 chips populating the card. You need to find an older SO-DIMM that doesn't have high density chips, which might eliminate current/recent production. I'd suggest checking ebay for pulled, older memory. I bought a lot of memory for Macs from a number of ebay sellers 15-20 years ago and never had a problem with compatibility. You just need to find non-parity, non-ECC, and non-high density chips. PC100 with CL2 would be my preference.