An index released Tuesday from the National Association shows that pending home sales, based on contracts signed in April, were up again for the third straight month. Is this a sure sign the mortgage and housing markets are headed back up? Not necessarily.

In the past the NAR’s Pending Home Sales Index has been a fairly accurate predictor of how many actual existing-home sales will take place, but in the past several months, that has not been true. There may actually be a lot more people signing contracts these days who are not able to go through with the sale.

Here’s why people want to be buying houses, according to NAR chief economist Lawrence Yun:

 “Housing affordability conditions have been at historic highs, but now the $8,000 first-time buyer tax credit is beginning to impact the market. Since first-time buyers must finalize their purchase by November 30 to get the credit, we expect greater activity in the months ahead, and that should spark more sales by repeat buyers.”

Additionally interest rates are incredibly low and prices keep plunging in many parts of the country.

Here’s why pending contracts are not going through as much these days, Yun says:

“Mortgage processing time has increased, it is taking many months to close on those homes requiring short sales with lender approval, and some sales are falling through at the last moment.”

Diana Olick on the CNBC website  writes that in addition to slow moving bank approvals, appraisals are also a factor.

“Many appraisals are now coming in lower than the contract price. Today’s already-iffy buyers aren’t willing to hang in there when they find out the house is worth less.”

So while pending home sales are always a good sign, they may not be the sign yet we are all looking for to indicate the healing of the housing market.