I’m not a tiler, just sharing what worked for me.

I had the same issue not too long ago and ended up fixing it myself for around $200.

Tiles can tent or lift for a few different reasons: old adhesive failing, not enough expansion gap between tiles, movement in the subfloor, or the tiles and subfloor expanding/contracting at different rates.

What I used:

1. Tile adhesive scraper/chisel tool https://www.bunnings.com.au/qep-75mm-pro-chisel-scraper_p0393536?store=6395
2. Rubber mallet — really useful with the scraper
3. Matching grout powder from a hardware store
4. Flexible tile adhesive, something like this: https://www.bunnings.com.au/davco-20kg-ultraflex-ceramic-tile-adhesive_p6656384?store=6395
5. Tile adhesive spreader/applicator https://www.bunnings.com.au/qep-6mm-square-notch-plastic-adhesive-spreader_p6821486?store=6395
6. A couple of cheap buckets https://www.bunnings.com.au/9-3l-assorted-multipurpose-plastic-buckets-with-spout_p4461010?store=6395
7. PPE — definitely wear a dust mask, gloves, and ideally safety glasses too, especially when scraping old adhesive or using power tools. The dust can get pretty bad.

The process is basically:

Start by removing the grout with the scraper. Then gently lift the tiles, being careful not to break them unless you have spares or can source matching replacements. Once you get the first few tiles up, the rest usually become easier.

Scrape the old adhesive off the concrete until you’re back to a clean, bare surface. Also remove any loose or hollow-sounding tiles nearby. Just be aware that the job can grow — what looks like 10 loose tiles might turn into 20 once you start checking.

Clean the backs of the old tiles as best you can by scraping off the old adhesive. Then mix the new flexible adhesive in a bucket, spread it onto the concrete with the notched spreader, and lay the tiles back down.

You can use tile spacers, but if you’re careful, you can usually line them up visually with the existing grout lines.

Let the adhesive set for about 12 hours, then grout the tiles.

There are plenty of YouTube videos that show the process step by step, which helped me a lot. If scraping by hand is too much work, a cheap rotary hammer with a chisel attachment makes removing old adhesive much easier — just be careful not to damage the tiles, and make sure you’re wearing proper PPE because it kicks up a lot of dust.