Why the credit is not showing in your 26AS
Credit for TDS is given on the basis of the statement the deductor files (Section 199 read with Rule 37BA). So the tax reaches your Form 26AS only when the buyer files their quarterly Form 27Q correctly, against your PAN. If they filed it against the wrong PAN, the credit is mechanically routed to whoever that PAN belongs to, and it will never appear against yours; if they did not file at all, there is nothing to show.
The buyer also has a duty to hand you a Form 16A certificate for the deduction (Section 203), which is your proof of the tax taken. When a buyer withholds the tax but then goes quiet on the paperwork, these are the two failures behind it: no Form 16A, and a Form 27Q that is wrong or missing. Both are the buyer's default, not yours, which is what the remedies below turn on.
The clean fix, and the protection if the buyer will not help
The straightforward route is a correction. The buyer files a revised Form 27Q on the TRACES portal with your correct PAN, and once it reprocesses, the credit flows into your Form 26AS and you claim it in the ordinary way. A practising CA usually gets this done by putting the buyer's exposure to them plainly, because the buyer is the one carrying the penalties.
Where the buyer simply will not cooperate, the law protects you directly. Under Section 205, once tax has been deducted at source, and property TDS under Section 195 is deductible at source, you cannot be asked to pay that tax again, whether or not the buyer deposited it or gave you a certificate. CBDT has instructed its officers not to enforce a demand that arises only because of the deductor's default. The honest caveat is that this is a protection you invoke, not an automatic credit in the software: the system will still flag a mismatch, and you defend it by producing proof of the deduction, the sale deed and the buyer's bank record showing they paid you the net amount, and citing Section 205.
The buyer is the one exposed, which is your leverage
It helps to know how much the buyer stands to lose, because it is what usually gets the correction filed. A buyer who deducted but did not deposit or file properly is an assessee in default and carries interest at 1% or 1.5% a month (Section 201), a late-filing fee of ₹200 a day for the delayed statement (Section 234E), and a penalty of ₹10,000 to ₹1,00,000 for filing the wrong PAN, because that counts as incorrect information (Section 271H).
So the buyer's incentive to fix a correction statement is real and immediate. A practising CA sets this out to the buyer, gets the 27Q corrected where possible, and where it is not, builds and files the Section 205 defence with your evidence so the credit is not denied and no demand is enforced against you.