Is the descent performance of the PMDG 737-800 accurate?
You haven't provided everything possible to really ascertain the issue noted, but based on what you have I've noticed the following things:


You'll find most 737s at 240-250 KIAS passing through 10000 feet, rather than the 220 knots that you were going at (while 220 KIAS is above minimum clean speed at normal landing weights, it's still slower than most)
Being heavy actually makes minimum drag and max L/D speeds higher, which means that for the same speed, you'll actually descend at less of a gradient than if you were lighter
The Boeing 737-800W has a idle descent ratio of about 1:20, which means that at 250 knots you're only going to be descending about 1200fpm -- the 737-800 (especially with winglets) is a very slick bird.


For your reference, you can see the distances required for an idle descent (.78/280/250), plus allowances for a straight-in approach:

|=====================================================|
|          |       |      |       DISTANCE (NM)       |
| PRESSURE |       |      |---------------------------|
| ALTITUDE | TIME  | FUEL |  LANDING WEIGHT (1000 KG) |
|   (FT)   | (MIN) | (KG) |  40  |  50  |  60  |  70  |
|----------|-------|------|------|------|------|------|
|   41000  |   27  |  340 |  102 |  119 | 133  |  142 |
|   39000  |   26  |  340 |   97 |  114 | 127  |  136 |
|   37000  |   25  |  330 |   92 |  108 | 121  |  130 |
|   35000  |   24  |  330 |   88 |  103 | 116  |  125 |
|   33000  |   24  |  320 |   84 |   99 | 111  |  120 |
|   31000  |   23  |  320 |   80 |   94 | 105  |  113 |
|   29000  |   22  |  310 |   75 |   88 |  98  |  106 |
|   27000  |   21  |  300 |   70 |   82 |  92  |   99 |
|   25000  |   20  |  300 |   66 |   77 |  86  |   92 |
|   23000  |   19  |  290 |   61 |   71 |  79  |   85 |
|   21000  |   18  |  280 |   57 |   66 |  73  |   78 |
|   19000  |   17  |  270 |   52 |   61 |  67  |   72 |
|   17000  |   15  |  250 |   48 |   55 |  61  |   65 |
|   15000  |   14  |  240 |   44 |   50 |  55  |   58 |
|   10000  |   11  |  200 |   30 |   34 |  37  |   39 |
|    5000  |    7  |  150 |   18 |   19 |  20  |   21 |
|    1500  |    4  |  110 |    9 |    9 |   9  |    9 |
|=====================================================|