You don't mention whether or not you went for a drive after you re-initialised the tyres. If not, then that would be the case as it requires driving for 5-10 minutes before they show the pressure.
As for replacements, someone on one of the Facebook groups was asking a similar question and I provided the following link (not affiliated to it). It's for a set of four TPMS sensors plus the all important programming pad (which is never mentioned in the other TPMS ads), as without it the sensors are useless. You just need to make sure and program them the same as your existing ones, including the correct frequency.
On the build sticker, you should have either 7K3 (433MHz) or 7K4 (315MHz).
https://www.ebay.co.uk/itm/133417324862
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
Sold! 
2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds